Semiconductor device and manufacturing method thereof

ABSTRACT

It is an object to provide a display device of which image display can be favorably recognized. Another object is to provide a manufacturing method of the display device with high productivity. Over a substrate, a pixel electrode that reflects incident light through a liquid crystal layer, a light-transmitting pixel electrode, and a structure whose side surface is covered with a reflective layer and which is positioned to overlap with the light-transmitting pixel electrode are provided. The structure is formed over a light-transmitting etching-stop layer, and the etching-stop layer remains below the structure as a light-transmitting layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/030,199, filed Feb. 18, 2011, now allowed, which claims the benefitof a foreign priority application filed in Japan as Serial No.2010-043185 on Feb. 26, 2010, both of which are incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device having a circuitformed using a transistor and a manufacturing method thereof. Forexample, the present invention relates to an electronic device on whichan electro-optical device typified by a liquid crystal display device ismounted as a component.

In this specification, a semiconductor device means all types of devicesthat can function by utilizing semiconductor characteristics, and anelectro-optic device, a semiconductor circuit, and electronic equipmentare all semiconductor devices.

2. Description of the Related Art

As a liquid crystal display device, an active matrix liquid crystaldisplay device in which pixel electrodes are arranged in matrix andtransistors are used as switching elements connected to respective pixelelectrodes in order to obtain a high-quality image, has attractedattention.

An active matrix liquid crystal display device, in which a transistorformed using a metal oxide for a channel formation region is used as aswitching element connected to each pixel electrode, has already beenknown (see Patent Document 1 and Patent Document 2).

It is known that an active matrix liquid crystal display device isclassified into two major types: transmissive type and reflective type.

In the transmissive liquid crystal display device, a backlight such as acold cathode fluorescent lamp is used and an optical modulationoperation of liquid crystal is utilized to choose one between the twostates: a state in which light from the backlight passes through liquidcrystal to be output to the outside of the liquid crystal display deviceand a state in which light is not output, whereby bright and dark imagesare displayed; further, image display is performed in combination ofthese.

Since the backlight is utilized in the transmissive liquid crystaldisplay device, it is difficult to recognize display in the environmentwith strong external light, for example, outdoors.

In the reflective liquid crystal display device, the optical modulationoperation of liquid crystal is utilized to choose between the twostates: a state in which external light, that is, incident light istransmitted through liquid crystal and reflected on a pixel electrode tobe output to the outside of the device and a state in which incidentlight is not output to the outside of the device, whereby bright anddark images are displayed; further, image display is performed incombination of them.

Compared to the transmissive liquid crystal display device, thereflective liquid crystal display device has the advantage of low powerconsumption since the backlight is not used; therefore, a demand for thereflective liquid crystal display device as a portable informationterminal has increased.

Since external light is utilized in the reflective liquid crystaldisplay device, the reflective liquid crystal display device is suitedfor image display in the environment with strong external light, forexample, outdoors. On the other hand, it is difficult to recognizedisplay when the liquid crystal display device is used in a dimenvironment, that is, in the environment with weak external light.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-096055

SUMMARY OF THE INVENTION

It is an object to provide a semiconductor device of which image displaycan be favorably recognized.

It is another object to provide a semiconductor device that can performimage display in both modes: a reflective mode where external light isutilized as an illumination light source; and a transmissive mode wherea backlight is used.

It is another object to provide a semiconductor device with bright andhigh-quality display without increasing power consumption.

It is another object to provide a manufacturing method of asemiconductor device with high productivity and less variation incharacteristics.

One embodiment of the invention disclosed in this specification achievesat least one of the above objects.

The semiconductor device includes a light-transmitting pixel and a pixelthat reflects incident light through a liquid crystal layer; therefore,display image can be performed in both modes: the reflective mode inwhich external light is utilized as an illumination light source; andthe transmissive mode in which a backlight is used as an illuminationlight source.

A plurality of structures for condensing light from the backlight isprovided in the light-transmitting pixel, whereby the amount oftransmitted light can be increased without increasing the luminance ofthe backlight.

In addition, by forming the structure over the oxide semiconductor, theoxide semiconductor functions as an etching-stop layer and damage to abottom layer in the formation process of the structure is reduced;therefore, a semiconductor device having high productivity and lessvariation in characteristics can be manufactured. As a material of theetching-stop layer, a conductive material, an insulating material, or asemiconductor material can be used as long as it is a light-transmittingmaterial that can withstand an etching process in the formation processof the structure. The etching-stop layer can be referred to as alight-transmitting layer or a light-condensing layer.

A semiconductor device of an embodiment of the present inventionincludes a structure comprising a side surface, a bottom surface and atop surface, a first pixel electrode including an electrode thatreflects incident light, and a second pixel electrode including anelectrode that has a light-transmitting property. The first pixelelectrode is connected to a first transistor, and the second pixelelectrode is connected to a second transistor. The side surface of thestructure is covered by a reflective layer, and the area of the bottomsurface of the structure is greater than that of the top surface of thestructure.

A transistor including an oxide semiconductor is used as each of thefirst transistor and the second transistor, whereby frequency of refreshoperation in displaying a still image can be reduced.

Each structure is formed so that the bottom surface of each structure isin contact with and on a light-transmitting layer. The structure haspreferably higher refractive index than the light-transmitting layer.

The structure is preferably formed using an organic resin such as apolyimide resin or an acrylic resin.

As a material of the reflective layer, a material with high reflectanceof visible light such as aluminum (Al) or silver (Ag), or an alloyincluding any of these is preferably used.

Each structure includes two inclined planes facing each other at a crosssection of each structure. An angle θR formed by the two inclined planesof the structure is less than 90°, preferably greater than or equal to10° and less than or equal to 60°.

The reflective pixel that reflects incident light includes a reflectiveelectrode, the reflective electrode includes a curving surface, and anangle θR at point where the reflective electrode is most curved at thecross section of the reflective electrode, formed by two inclined planesfacing each other is greater than or equal to 90°, preferably greaterthan or equal to 100° and less than or equal to 120°.

An embodiment of the present invention is a manufacturing method of asemiconductor device including the steps of: forming an insulating layerover a substrate; forming a light-transmitting etching-stop layer overthe insulating layer; forming an organic resin layer over theetching-stop layer; etching the organic resin layer selectively toexpose the etching-stop layer; and forming a plurality of structures byetching the exposed etching-stop layer.

An embodiment of the present invention is a manufacturing method of asemiconductor device including the steps of: forming a first insulatinglayer over a substrate; forming an etching-stop layer over the firstinsulating layer; forming a second insulating layer over theetching-stop layer; forming a resist mask over the second insulatinglayer; and forming a plurality of structures by etching the resist maskwhile etching the second insulating layer.

An oxide semiconductor is preferably used for the etching-stop layerbecause the selectivity in formation of the structures can be increased.

The second insulating layer is preferably formed using an organic resinsuch as a polyimide resin or an acrylic resin.

The second insulating layer is preferably etched by a dry etchingmethod.

The etching-stop layer that is exposed by the etching of the secondinsulating layer is preferably removed by a wet etching method.

A semiconductor device with bright and high-quality display can beprovided. A semiconductor device with high productivity and lessvariation in characteristics can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C are diagrams each illustrating a cross-sectionalstructure of a semiconductor device;

FIGS. 2A and 2B are diagrams each illustrating a cross-sectionalstructure of a semiconductor device;

FIG. 3 is a diagram illustrating a plane structure of a semiconductordevice;

FIGS. 4A to 4C are cross-sectional views illustrating a manufacturingprocess of the semiconductor device;

FIGS. 5A to 5C are cross-sectional views illustrating the manufacturingprocess of the semiconductor device;

FIGS. 6A and 6B are cross-sectional views illustrating the manufacturingprocess of the semiconductor device;

FIGS. 7A to 7C are cross-sectional views illustrating the manufacturingprocess of the semiconductor device.

FIGS. 8A to 8C are cross-sectional views illustrating the manufacturingprocess of the semiconductor device;

FIGS. 9A to 9C are cross-sectional views illustrating the manufacturingprocess of the semiconductor device;

FIGS. 10A and 10B are cross-sectional views illustrating themanufacturing process of the semiconductor device;

FIGS. 11A to 11D are diagrams each illustrating one mode of a transistorthat can be applied to a semiconductor device;

FIG. 12 is a perspective view of a liquid crystal module;

FIG. 13 is a block diagram illustrating a structure of a semiconductordevice;

FIG. 14 is a block diagram illustrating a structure of a semiconductordevice;

FIG. 15 is a diagram illustrating a structure of a driver circuit and apixel of a semiconductor device;

FIG. 16 is a diagram illustrating a structure of a driver circuit and apixel of a semiconductor device;

FIG. 17 is a timing chart illustrating operation of a semiconductordevice;

FIGS. 18A and 18B are timing charts each illustrating operation of adisplay control circuit of a semiconductor device;

FIG. 19 is a schematic diagram illustrating the frequency of writing ofimage signals per frame period in a period for displaying a moving imageand a period for displaying a still image of a semiconductor device; and

FIGS. 20A and 20B are diagrams illustrating an example of an electronicdevice including a semiconductor device.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the drawings. Note that the present inventionis not limited to the following description and it will be readilyappreciated by those skilled in the art that modes and details can bemodified in various ways. Further, the invention should not be construedas being limited to the description in the following embodiments.

A transistor is a kind of semiconductor elements and can achieveamplification of a current or a voltage, switching operation forcontrolling conduction or non-conduction, or the like. A transistor inthis specification includes an insulated-gate field effect transistor(IGFET) and a thin film transistor (TFT).

Note that the position, the size, the range, or the like of eachstructure illustrated in drawings and the like is not accuratelyrepresented in some cases for easy understanding. Therefore, thedisclosed invention is not necessarily limited to the position, size,range, or the like as disclosed in the drawings and the like. Inaddition, in a circuit diagram, “OS” is written beside a transistor inorder to indicate that the transistor includes an oxide semiconductor.

In this specification and the like, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not mean limitation of the number ofcomponents.

In this specification and the like, the terms “over” and “below” do notnecessarily mean “directly on” and “directly below”, respectively, inthe description of a physical relationship between components. Forexample, the expression “a gate electrode over a gate insulating layer”can mean the case where there is an additional component between thegate insulating layer and the gate electrode.

Further, in this specification, a source electrode and a drain electrodeof a transistor may be interchanged with each other depending on thestructure, the operating condition, or the like of the transistor;therefore, it is difficult to define which is a source electrode or adrain electrode in electrodes of the transistor other than a gateelectrode. Therefore, the terms “source electrode” and “drain electrode”can be switched in this specification.

In this specification and the like, the term such as “electrode” or“wiring” does not limit a function of a component. For example, an“electrode” is sometimes used as part of a “wiring”, and vice versa.Furthermore, the term “electrode” or “wiring” can include the case wherea plurality of “electrodes” or “wirings” are formed in an integratedmanner.

Note that when it is explicitly described that “A and B are connected”,the case where A and B are electrically connected, the case where A andB are functionally connected, and the case where A and B are directlyconnected are included therein.

Embodiment 1

In this embodiment, a pixel structure which enables an increase in theamount of reflected light and transmitted light per one pixel in asemi-transmissive liquid crystal display device and a manufacturingmethod thereof will be described with reference to FIGS. 1A to 1C, FIGS.2A and 2B, FIG. 3, FIGS. 4A to 4C, FIGS. 5A to 5C, FIGS. 6A and 6B,FIGS. 7A to 7C, FIGS. 8A to 8C, FIGS. 9A to 9C, and FIGS. 10A and 10B.

FIGS. 1A to 1C and FIGS. 2A and 2B illustrate cross-sectional structuresof a pixel described in this embodiment. FIG. 3 illustrates a planestructure of the pixel described in this embodiment. FIGS. 1A to 1Cillustrate cross-sectional structures of a portion along S1-S2, aportion along T1-T2, and a portion along U1-U2, respectively, denoted bydashed lines in FIG. 3. FIG. 2A is an enlarged view of a portion 880 inFIG. 1A. FIG. 2B is an enlarged view of a portion 881 in FIG. 1A.

The pixel described in this embodiment includes a transparent electrode823 and a reflective electrode 825 as pixel electrodes. The transparentelectrode 823 and a reflective layer 821 are connected to a drainelectrode 857 of a transistor 851 through a contact hole 855 provided inan insulating layer 828. Note that the transparent electrode 823 can bedirectly connected to the drain electrode 857. The drain electrode 857overlaps with a capacitor wiring 853 with the insulating layer 827provided therebetween so that a storage capacitor 871 is formed (seeFIG. 1B and FIG. 3).

A gate electrode 858 of the transistor 851 is connected to a wiring 852,and a source electrode 856 of the transistor 851 is connected to awiring 854 (see FIG. 3). The transistor 851 includes a semiconductorlayer 859. As the semiconductor layer 859, for example, an amorphoussemiconductor, a microcrystalline semiconductor, or polycrystallinesemiconductor containing silicon or germanium, or the like can be used.Alternatively, an organic semiconductor formed by a printing method oran inkjet method can be used. With the use of an oxide semiconductor,the transistor 851 can have an extremely low off-state current, so thatdisplay quality can be increased.

The reflective electrode 825 is connected to a drain electrode 867 of atransistor 861 through a contact hole 865 provided in the insulatinglayer 828 and an organic resin layer 822 (see FIG. 1C). The drainelectrode 867 overlaps with a capacitor wiring 863 with the insulatinglayer 827 provided therebetween to form a storage capacitor 872.

A gate electrode 868 of the transistor 861 is connected to a wiring 862,and a source electrode 866 of the transistor 861 is connected to awiring 864. The transistor 861 includes a semiconductor layer 869. Asthe semiconductor layer 869, for example, an amorphous semiconductor, amicrocrystalline semiconductor, or polycrystalline semiconductorcontaining silicon or germanium, or the like can be used. Alternatively,an organic semiconductor formed by a printing method or an inkjet methodcan be used. With the use of an oxide semiconductor, the transistor 861can have an extremely low off-state current, so that display quality canbe increased.

External light is reflected by the reflective electrode 825, so that thepixel electrode can function as a pixel electrode of a reflective liquidcrystal display device (a reflective mode). A plurality of openings 826is provided in the reflective electrode 825 (see FIG. 1A or FIG. 3). Ineach opening 826, the reflective electrode 825 does not exist, and thetransparent electrode 823 is exposed (see FIG. 1A). Light from abacklight is transmitted through the structure 820 from the opening 826,so that the transparent electrode 823 can function as a pixel electrodeof a transmissive liquid crystal display device (a transmissive mode).

In the semi-transmissive liquid crystal display device described in thisembodiment, the reflective electrode 825 and the transparent electrode823 are electrically isolated by the organic resin layer 822. Thepotential applied to the transparent electrode 823 can be controlled bythe transistor 851, and the potential applied to the reflectiveelectrode 825 can be controlled by the transistor 861; therefore, thepotential of the reflective electrode 825 and the potential of thetransparent electrode 823 can be controlled independently. Accordingly,for example, when the semi-transmissive liquid crystal display devicefunctions as a transmissive liquid crystal display device, the imagedisplayed using a liquid crystal display over the reflective electrodecan be black.

Reflected light 832 is external light reflected by the reflectiveelectrode 825 (see FIG. 2A). The top surface of the organic resin layer822 is a curving surface with an uneven shape (see FIGS. 1A to 1C). Thereflective electrode 825 also has the curving surface with an unevenshape; thus, the area of the reflective region can be increased, andglare of the outside light can be reduced so that visibility of thedisplayed image can be improved.

In the cross-sectional shape, the angle θR at a point where thereflective electrode 825 having a curving surface is most curved, formedby two inclined planes facing each other may be greater than or equal to90°, preferably greater than or equal to 100° and less than or equal to120° (see FIG. 2A).

The structure 820 is formed in a lower layer of the opening 826 tooverlap with the opening 826 (see FIG. 1A).

The structure 820 includes a backlight exit 841 on the opening 826 side(a top surface side of the structure 820) and a backlight entrance 842on a substrate 800 side (a bottom surface side of the structure 820).The area of the backlight entrance 842 (the area of the bottom surfaceof the structure 820) is larger than the area of the backlight exit 841(the area of the top surface of the structure 820). The reflective layer821 is formed on the side surfaces of the structure 820 (surfaces otherthan the backlight exit 841 and the backlight entrance 842). As amaterial for the structure 820, an inorganic material or an organicresin material can be used as long as it is a light-transmittingmaterial such as silicon oxide (SiO_(x)), silicon nitride (SiN_(x)),silicon oxynitride (SiNO), an acrylic resin, a polyimide resin, asiloxane resin, phosphosilicate glass (PSG), or borophosphosilicateglass (BPSG). The reflective layer 821 can be formed using a materialwith high reflectance of visible light such as aluminum (Al) or silver(Ag), or an alloy including any of these. An etching-stop layer 829 isprovided between the structure 820 and the insulating layer 828 (seeFIG. 1A and FIG. 2B).

Light emitted from the backlight enters the structure 820 through thebacklight entrance 842 as incident light 831. Some of the incident light831 is directly transmitted through the backlight exit 841, some isreflected toward the backlight exit 841 by the reflective layer 821, andsome is further reflected to return to the backlight entrance 842.

Here, according to the shape of a cross section of the structure 820from a direction perpendicular to the substrate 800, side surfaces onright and left facing each other are inclined surfaces. The angle θTformed by the side surfaces facing each other is made to be less than90°, preferably greater than or equal to 10° and less than or equal to60°, so that the incident light 831 incident from the backlight entrance842 can be guided efficiently to the backlight exit 841. The structure820 has a function of condensing the incident light 831 incident fromthe backlight entrance 842 and guiding the light to the backlight exit841.

The etching-stop layer 829 is used to prevent the insulating layer 828in the bottom layer from being removed by etching in a process offormation of the structure 820, and details thereof will be described inthe description of a manufacturing process below. As the etching-stoplayer 829, a material that is not easily etched (a high etchingselectivity is obtained) in the process of the formation of thestructure 820 is used. As the etching-stop layer 829, alight-transmitting material is used because the etching-stop layer 829remains between the structure 820 and the insulating layer 828. For thatreason, the etching-stop layer can be referred to as alight-transmitting layer.

Here, when a material with a higher refractive index than a material ofthe insulating layer 828 is used for the etching-stop layer 829, inaccordance with Snell's law, the incident light 831 can be guidedefficiently to the backlight exit 841. This is because, among theincident light 831 incident from the insulating layer 828 to theetching-stop layer 829, incident light with an incidence angle i (anangle formed by the incident light and a normal to the interface betweenthe insulating layer 828 and the etching-stop layer 829) is guided tothe structure 820 as refractive light with a refraction angle j (anangle formed by the refractive light a normal to the interface betweenthe insulating layer 828 and the etching-stop layer 829) which is ashallower than the angle i. That is, the etching-stop layer 829 canfunction as a light-condensing layer. In addition, when a material witha higher refractive index than that of the etching-stop layer 829 isused for the structure 820, the above effect can be further increased.

In a conventional semi-transmissive liquid crystal display device, whenthe area of an electrode in a pixel electrode, functioning as areflective electrode is SR and the area of an electrode in a pixelelectrode, functioning as a transmissive electrode (the area of theopening 826) is ST, the proportion of the total area of both electrodesis 100% (SR+ST=100%) of an effective pixel area. In thesemi-transmissive liquid crystal display device having a pixel structuredescribed in this embodiment, since the area ST of the electrodefunctioning as a transmissive electrode corresponds to the area of thebacklight entrance 842, the amount of transmitted light can be increasedwithout increasing the area of the opening 826 or the luminance of thebacklight. In other words, the proportion of the total area of bothelectrodes in appearance can be 100% or more (SR+ST is 100% or more)comparing an effective pixel area.

With this embodiment, a semi-transmissive liquid crystal display devicewith bright and high-quality display can be obtained without increasingpower consumption.

Next, a manufacturing process of the semiconductor device described inthis embodiment will be described with reference to FIGS. 4A to 4C,FIGS. 5A to 5C, FIGS. 6A and 6B, FIGS. 7A to 7C, FIGS. 8A to 8C, FIGS.9A to 9C, and FIGS. 10A and 10B.

First, a conductive layer is formed over the light-transmittingsubstrate 800, and then, the gate electrode 858 and the capacitor wiring853 are formed by a first photolithography step. Note that a resist maskmay be formed by an inkjet method. Formation of the resist mask by aninkjet method needs no photomask; thus, manufacturing cost can bereduced.

As the substrate 800, a glass substrate, a ceramic substrate, a quartzsubstrate, a sapphire substrate, a plastic substrate having heatresistance high enough to withstand a treatment temperature in thismanufacturing process, or the like can be used. Alternatively,crystallized glass or the like may be used.

When the temperature of the heat treatment performed later is high, asubstrate having a strain point of 730° C. or higher is preferably usedas the glass substrate. As the glass substrate, for example, analkali-free glass substrate of barium borosilicate glass,aluminoborosilicate glass, aluminosilicate glass, or the like may beused. Note that when the glass substrate contains more barium oxide(BaO) than boron oxide (B₂O₃), a more practical heat-resistant glass canbe obtained. Therefore, a glass substrate containing BaO and B₂O₃ sothat the amount of BaO is larger than that of B₂O₃ is preferably used.

Further, an insulating layer as a base insulating layer may be providedbetween the substrate 800 and the gate electrode 858 and between thesubstrate 800 and the capacitor wiring 853. The base insulating layerhas a function of preventing diffusion of an impurity element from thesubstrate 800, and can be formed with a single-layer or stacked-layerstructure using one or more of a silicon nitride layer, a silicon oxidelayer, a silicon nitride oxide layer, and a silicon oxynitride layer.

When a halogen element such as chlorine or fluorine is contained in thebase insulating layer, the function of preventing diffusion of animpurity element from the substrate 800 can be further increased. Thepeak of the concentration of a halogen element to be contained in thebase insulating layer is preferably greater than or equal to 1×10¹⁵/cm³and less than or equal to 1×10²⁰/cm³ when measured by secondary ion massspectrometry (SIMS).

The conductive layers for forming the gate electrode 858 and thecapacitor wiring 853 can be formed with a single-layer or stacked-layerstructure using a metal material such as molybdenum, titanium, tantalum,tungsten, aluminum, copper, neodymium, or scandium, or an alloy materialcontaining any of these materials as its main component. For example,the metal conductive film may have a three-layer structure in which analuminum layer is stacked over a titanium layer and a titanium layer isstacked over the aluminum layer, or a three layer structure in which analuminum layer is stacked over a molybdenum layer and a molybdenum layeris stacked over the aluminum layer. Further, an aluminum material towhich an element that prevents generation of a hillock or a whisker inan aluminum layer (such as silicon, neodymium, or scandium) is added canbe used for an aluminum layer, so that heat resistance can be increased.

The etching of the conductive layer may be performed by either a dryetching method or a wet etching method. For dry etching, an excitationmethod such as an inductively coupled plasma (ICP) etching method usinga gas containing chlorine or fluorine as an etching gas can be used.

The thickness of the gate electrode 858 and the capacitor wiring 853 ispreferably greater than or equal to 50 nm and less than or equal to 500nm. The end portions of the gate electrode 858 and the capacitor wiring853 are tapered, whereby coverage with the insulating layer 827 formedlater can be improved and disconnection can be prevented. In thisembodiment, a 100-nm-thick tungsten film is used as a conductive filmforming the gate electrode 858 and the capacitor wiring 853 (see FIG.4A).

Next, the insulating layer 827 is formed over the gate electrode 858 andthe capacitor wiring 853. The insulating layer 827 can be formed with asingle-layer structure or a stacked-layer structure using any of asilicon oxide layer, a silicon nitride layer, a silicon oxynitridelayer, a silicon nitride oxide layer, an aluminum oxide layer, analuminum nitride layer, an aluminum oxynitride layer, an aluminumnitride oxide layer, and a hafnium oxide layer by a plasma CVD method, asputtering method, or the like.

For example, a silicon oxynitride layer may be formed using SiH₄,oxygen, and nitrogen as deposition gases by a plasma CVD method. When asilicon oxide film is formed by a sputtering method, a silicon target ora quartz target is used as a target and oxygen or a mixed gas of oxygenand argon is used as a sputtering gas.

The thickness of the insulating layer 827 is greater than or equal to100 nm and less than or equal to 500 nm; when the insulating layer 827is formed using a stacked layer, for example, a first insulating layerwith a thickness of greater than or equal to 50 nm and less than orequal to 200 nm and a second insulating layer with a thickness ofgreater than or equal to 5 nm and less than or equal to 300 nm arestacked. In this embodiment, the insulating layer 827 is formed of a100-nm-thick silicon oxynitride layer (see FIG. 4B).

Note that when an oxide semiconductor that is made to be an i-type orsubstantially i-type by removing impurities is used for thesemiconductor layer 859 formed later, the insulating layer 827 that isin contact with the highly purified oxide semiconductor needs to havehigher quality. This is because such a highly purified oxidesemiconductor is highly sensitive to an interface state and interfaceelectric charge.

For example, a high-density plasma CVD method using microwaves (e.g., afrequency of 2.45 GHz) is preferably adopted because an insulating layercan be dense and can have high withstand voltage and high quality. Thehighly-purified oxide semiconductor and the high-quality gate insulatinglayer are in close contact with each other, whereby the interface statedensity can be reduced to obtain favorable interface characteristics.

Needless to say, another film formation method such as a sputteringmethod or a plasma CVD method can be employed as long as a high-qualityinsulating layer can be formed as a gate insulating layer. Further, aninsulating layer whose film quality as a gate insulating layer andcharacteristics of the interface between the insulating layer and anoxide semiconductor are improved by heat treatment that is performedafter formation of the insulating layer may be formed. In any case, anyinsulating layer may be used as long as the insulating layer hascharacteristics of enabling a reduction in interface state density ofthe interface between the insulating layer and an oxide semiconductorand formation of a favorable interface as well as having favorable filmquality as a gate insulating layer.

Next, over the insulating layer 827, a semiconductor layer having athickness greater than or equal to 2 nm and less than or equal to 200nm, preferably greater than or equal to 5 nm and less than or equal to30 nm is formed.

The semiconductor layer can be formed of a semiconductor layer with anamorphous, microcrystalline, or polycrystalline crystal structure by aknown method such as a CVD method, a sputtering method, or a laserannealing method. For example, a layer of an amorphous semiconductor ora microcrystalline semiconductor can be formed using a deposition gasdiluted with hydrogen by a plasma CVD method. As a deposition gas, a gascontaining silicon or germanium can be used. As a deposition gascontaining silicon, silane (SiH₄), disilane (Si₂H₆), dichlorosilane(SiH₂Cl₂), trichlorosilane (SiHCl₃), silicon chloride (SiCl₄), siliconfluoride (SiF₄), or the like can be used. As a deposition gas containinggermanium, germane (GeH₄), digermane (Ge₂H₆), germane fluoride (GeF₄),or the like can be used.

A polycrystalline semiconductor can be formed by forming an amorphoussemiconductor or a microcrystalline semiconductor and then subjectingthe semiconductor to heat treatment at 600° C. or more, RTA treatment,or laser light irradiation. Crystallization by RTA treatment or laserlight irradiation, by which a semiconductor layer can be instantaneouslyheated, is particularly effective in the case of forming apolycrystalline semiconductor over a substrate having a low strainpoint.

Alternatively, the oxide semiconductor can be formed by a sputteringmethod in a rare gas (typically, argon) atmosphere, an oxygenatmosphere, or a mixed atmosphere of a rare gas and oxygen. Note that apulse direct current (DC) power source is preferably used as a powersource because powder substances (also referred to as particles or dust)generated in film formation can be reduced and the film thickness can beuniform.

As an oxide semiconductor used for the oxide semiconductor film, thefollowing can be used: an In—Sn—Ga—Zn—O-based oxide semiconductor whichis a four-component metal oxide; an In—Ga—Zn—O-based oxidesemiconductor, an In—Sn—Zn—O-based oxide semiconductor, anIn—Al—Zn—O-based oxide semiconductor, a Sn—Ga—Zn—O-based oxidesemiconductor, an Al—Ga—Zn—O-based oxide semiconductor, or aSn—Al—Zn—O-based oxide semiconductor which are three-component metaloxides; an In—Zn—O-based oxide semiconductor, a Sn—Zn—O-based oxidesemiconductor, an Al—Zn—O-based oxide semiconductor, a Zn—Mg—O-basedoxide semiconductor, a Sn—Mg—O-based oxide semiconductor, anIn—Mg—O-based oxide semiconductor, or an In—Ga—O-based oxidesemiconductor which are two-component metal oxides; or an In—O-basedoxide semiconductor, a Sn—O-based oxide semiconductor, or a Zn—O-basedoxide semiconductor. Further, SiO₂ may be contained in the above oxidesemiconductor. Here, for example, an In—Ga—Zn—O-based oxidesemiconductor is an oxide including at least In, Ga, and Zn, and thereis no particular limitation on the composition ratio thereof. Further,the In—Ga—Zn—O-based oxide semiconductor may contain an element otherthan In, Ga, and Zn.

For the oxide semiconductor, a thin film represented by the chemicalformula, InMO₃(ZnO)_(m) (m>0), can be used. Here, M represents one ormore metal elements selected from Ga, Al, Mn, and Co. For example, M canbe Ga, Ga and Al, Ga and Mn, Ga and Co, or the like.

When an In—Zn—O-based material is used as the oxide semiconductor, atarget used has a composition ratio of In:Zn=50:1 to 1:2 in an atomicratio (In₂O₃:ZnO=25:1 to 1:4 in a molar ratio), preferably In:Zn=20:1 to1:1 in an atomic ratio (In₂O₃:ZnO=10:1 to 1:2 in a molar ratio), furtherpreferably In:Zn=15:1 to 1.5:1 (In₂O₃:ZnO=15:2 to 3:4 in a molar ratio).For example, when a target used for forming the In—Zn—O-based oxidesemiconductor has a composition ratio of In:Zn:O=x:y:z in an atomicratio, Z>(1.5x+y).

In this embodiment below, an example in which the oxide semiconductor isformed using an In—Ga—Zn—O-based oxide semiconductor target by asputtering method as a semiconductor layer will be described.

Note that the oxide semiconductor used for the semiconductor layer inthis embodiment is an i-type (intrinsic) oxide semiconductor or asubstantially i-type (intrinsic) oxide semiconductor. The i-type(intrinsic) oxide semiconductor or substantially i-type (intrinsic)oxide semiconductor is obtained in such a manner that hydrogen, which isan n-type impurity, is removed from an oxide semiconductor, and theoxide semiconductor is highly purified so that impurities that are notmain components of the oxide semiconductor are contained as little aspossible. In other words, a feature is that a purified i-type(intrinsic) semiconductor or a semiconductor close thereto is obtainednot by adding an impurity but by removing an impurity such as hydrogenor water as much as possible. Thus, an oxide semiconductor layerincluded in the transistor described in this embodiment is highlypurified to become electrically i-type (intrinsic).

In addition, a highly purified oxide semiconductor includes extremelyfew carriers (close to zero), and the carrier density thereof is lowerthan 1×10¹⁴/cm³, preferably lower than 1×10¹²/cm³, further preferablylower than 1×10¹¹/cm³.

Since the oxide semiconductor includes extremely few carriers, off-statecurrent of a transistor can be reduced. The smaller the amount ofoff-state current is, the better.

Specifically, in a transistor including the above-described oxidesemiconductor layer, the off-state current density per channel width of1 μm at room temperature can be reduced to less than or equal to 10aA/μm (1×10⁻¹⁷ A/μm), further to less than or equal to 1 aA/μm (1×10⁻¹⁸A/μm), still further to less than or equal to 10 zA/μm (1×10⁻²⁰ A/μm).Although it is difficult to achieve such a low off-state current with atransistor including a general silicon semiconductor, it can be achievedwith a transistor including an oxide semiconductor that is processedunder an appropriate condition and has a large energy gap of 3.0 eV to3.5 eV.

By using a transistor with an extremely small current value in an offstate (off-state current) as a transistor in a pixel portion, a refreshoperation in displaying a still image can be performed with a smallnumber of times of writing of image data.

In addition, in the transistor including the oxide semiconductor layer,the temperature dependence of on-state current is hardly observed, andthe off-state current remains extremely small.

As the metal oxide target used for forming an oxide semiconductor layerwith a sputtering method, for example, a metal oxide target having acomposition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:1[molar ratio] can be used.There is no limitation to the material and the composition of the abovetarget, for example, a target having a composition ratio ofIn₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio], In₂O₃:Ga₂O₃:ZnO=2:2:1 [molarratio], or In₂O₃:Ga₂O₃:ZnO=1:1:4 [molar ratio] may be used. The fillingrate of the metal oxide target is greater than or equal to 90% and lessthan or equal to 100%, preferably greater than or equal to 95% and lessthan or equal to 99.9%. With the use of the metal oxide target with highfilling rate, a dense oxide semiconductor film is formed.

It is preferable that a high-purity gas in which impurities such ashydrogen, water, a hydroxyl group, and hydride are removed be used asthe sputtering gas for the formation of the oxide semiconductor film.

In order that hydrogen, hydroxyl group, and moisture are contained aslittle as possible in the insulating layer 827 and the semiconductorlayer, it is preferable that as pretreatment before film formation of anoxide semiconductor, the substrate 800 over which the gate electrode 858and the capacitor wiring 853 are formed, or the substrate 800 over whichlayers up to and including the insulating layer 827 are formed bepreheated in a preheating chamber of a sputtering apparatus, so thatimpurities such as hydrogen or moisture absorbed onto the substrate areeliminated and removed. As an exhaustion unit provided for thepreheating chamber, a cryopump is preferable. Note that this preheatingtreatment can be omitted. In addition, before the insulating layer 828is formed, the preheating may similarly be performed on the substrate800 over which layers up to and including the source electrode 856 andthe drain electrode 857 are formed.

When the oxide semiconductor film is deposited, the substrate is held ina deposition chamber kept under reduced pressure, and the substratetemperature is set to temperatures higher than or equal to 100° C. andlower than or equal to 600° C., preferably higher than or equal to 200°C. and lower than or equal to 400° C. By heating the substrate duringdeposition, the impurity concentration in the oxide semiconductor layerthat is formed can be reduced. In addition, damage by sputtering can bereduced. Then, residual moisture in the deposition chamber is removed, asputtering gas from which hydrogen and moisture are removed isintroduced, and the above-described target is used, so that the oxidesemiconductor film is formed. In order to remove the residual moisturein the deposition chamber, an entrapment vacuum pump, for example, acryopump, an ion pump, or a titanium sublimation pump is preferablyused. The evacuation unit may be a turbo pump provided with a cold trap.In the deposition chamber which is evacuated with the cryopump, ahydrogen atom, a compound containing a hydrogen atom, such as water(H₂O), (further preferably, also a compound containing a carbon atom),and the like are evacuated, whereby the concentration of an impurity inthe oxide semiconductor layer formed in the deposition chamber can bereduced.

Further, the insulating layer 827 and the oxide semiconductor film maybe formed successively without exposure to the air. Film formationwithout exposure to air makes it possible to obtain an interface betweenthe stacked layers, which is not contaminated by atmospheric componentsor impurity elements floating in the air, such as water or hydrocarbon.Therefore, variation in characteristics of the transistor can bereduced.

Next, the semiconductor layer 859 is formed by processing thesemiconductor layer into an island shape by a second photolithographystep (see FIG. 4C). A resist mask for forming the island-shapedsemiconductor layer may be formed by an inkjet method. Formation of theresist mask by an inkjet method needs no photomask; thus, manufacturingcost can be reduced.

Note that the etching of the semiconductor layer here may be performedby either one or both of a dry etching method and a wet etching method.As an etchant used for wet etching of the oxide semiconductor film, asolution obtained by mixing phosphoric acid, acetic acid, and nitricacid, an ammonia peroxide mixture, or the like can be used, for example.As the ammonia peroxide mixture, specifically, an aqueous solution inwhich oxygenated water of 31 wt %, ammonia water of 28 wt %, and waterare mixed at a volume ratio of 2:1:1 is used. Alternatively, ITO-07N(produced by KANTO CHEMICAL CO., INC.) may be used.

Next, a first heat treatment is performed on the semiconductor layer859. The oxide semiconductor layer can be dehydrated or dehydrogenatedby the first heat treatment. The temperature of the first heat treatmentis higher than or equal to 400° C. and lower than or equal to 750° C.,or higher than or equal to 400° C. and lower than the strain point ofthe substrate. Here, the substrate is introduced into an electricfurnace which is one of heat treatment apparatuses, heat treatment isperformed on the semiconductor layer 859 in a nitrogen atmosphere at450° C. for one hour, and then, the semiconductor layer 859 is notexposed to the air so that entry of water and hydrogen into the oxidesemiconductor layer is prevented; thus, the dehydrated or dehydrogenatedsemiconductor layer 859 is obtained.

Note that a heat treatment apparatus is not limited to an electricfurnace, and may include a device for heating an object to be processedby heat conduction or heat radiation from a heating element such as aresistance heating element. For example, an RTA (rapid thermal anneal)apparatus such as a GRTA (gas rapid thermal anneal) apparatus or an LRTA(lamp rapid thermal anneal) apparatus can be used. An LRTA apparatus isan apparatus for heating an object to be processed by radiation of light(an electromagnetic wave) emitted from a lamp such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high-pressuresodium lamp, or a high-pressure mercury lamp. A GRTA apparatus is anapparatus for heat treatment using a high-temperature gas. As the hightemperature gas, an inert gas that does not react with an object to beprocessed by heat treatment, such as nitrogen or a rare gas like argon,is used.

For example, as the first heat treatment, GRTA may be performed asfollows: the substrate is transferred and put in an inert gas heated ata high temperature higher than or equal to 650° C. and lower than orequal to 700° C., is heated for several minutes, and is transferred andtaken out of the inert gas heated at the high temperature.

Note that in the first heat treatment, it is preferable that water,hydrogen, and the like be not contained in nitrogen or a rare gas suchas helium, neon, or argon. It is preferable that the purity of nitrogenor the rare gas such as helium, neon, or argon which is introduced intoa heat treatment apparatus be higher than or equal to 6N (99.9999%),preferably higher than or equal to 7N (99.99999%) (that is, the impurityconcentration is lower than or equal to 1 ppm, preferably lower than orequal to 0.1 ppm).

Further, after the semiconductor layer 859 is heated in the first heattreatment, a high-purity oxygen gas, a high-purity N₂O gas, or anultra-dry air (the dew point is lower than or equal to −40° C.,preferably lower than or equal to −60° C.) may be introduced into thesame furnace. It is preferable that water, hydrogen, and the like be notcontained in an oxygen gas or a N₂O gas. It is preferable that thepurity of the oxygen gas or the N₂O gas which is introduced into theheat treatment apparatus be higher than or equal to 6N (99.9999%),preferably higher than or equal to 7N (99.99999%) (i.e., theconcentration of impurities in the oxygen gas or the N₂O gas is lowerthan or equal to 1 ppm, preferably lower than or equal to 0.1 ppm).Oxygen which is a main component included in the oxide semiconductor andwhich has been reduced because of the step of removing impurities bydehydration or dehydrogenation is supplied by the action of the oxygengas or the N₂O gas, so that the semiconductor layer 859 can be ahighly-purified and electrically i-type (intrinsic) semiconductor layer.

The first heat treatment may be performed on the oxide semiconductorfilm that has not yet been processed into the island-shapedsemiconductor layer 859. In that case, the substrate is taken out fromthe heat apparatus after the first heat treatment, and then aphotolithography step is performed.

Note that the first heat treatment may be performed at any of thefollowing timings in addition to the above timing as long as it isperformed after deposition of the oxide semiconductor layer: after asource electrode layer and a drain electrode layer are formed over theoxide semiconductor layer and after an insulating layer is formed overthe source electrode layer and the drain electrode layer.

Further, the step of forming the contact hole in the insulating layer827 by a third photolithography step may be performed either before orafter the first heat treatment is performed.

In addition, as the oxide semiconductor film, an oxide semiconductorfilm having a crystal region with a large thickness (a single crystalregion), that is, a crystal region which is c-axis-alignedperpendicularly to a surface of the film may be formed by performingdeposition twice and heat treatment twice, even when any of an oxide, anitride, a metal, or the like is used for a material of a basecomponent. For example, a first oxide semiconductor film with athickness of greater than or equal to 3 nm and less than or equal to 15nm is deposited, and the first heat treatment is performed in anitrogen, an oxygen, a rare gas, or a dry air atmosphere at atemperature higher than or equal to 450° C. and lower than or equal to850° C. or preferably higher than or equal to 550° C. and lower than orequal to 750° C., so that a first oxide semiconductor film having acrystal region (including a plate-like crystal) in a region including asurface is formed. Then, a second oxide semiconductor film which has alarger thickness than the first oxide semiconductor film is formed, anda second heat treatment is performed at higher than or equal to 450° C.and lower than or equal to 850° C. or preferably higher than or equal to600° C. and lower than or equal to 700° C., so that crystal growthproceeds upward with the use of the first oxide semiconductor film as aseed of the crystal growth and the whole second oxide semiconductor filmis crystallized. In such a manner, the oxide semiconductor film having acrystal region having a large thickness may be formed.

Next, a conductive layer is formed over the insulating layer 827 and thesemiconductor layer 859 and selectively etched by a fourthphotolithography step, so that the source electrode 856 and the drainelectrode 857 are formed. Note that a resist mask may be formed by aninkjet method. Formation of the resist mask by an inkjet method needs nophotomask; thus, manufacturing cost can be reduced.

The conductive layer can be formed using the material similar to that ofthe gate electrode 858 and the capacitor wiring 853. The thickness ofthe source electrode 856 and the drain electrode 857 is preferablygreater than or equal to 100 nm and less than or equal to 10000 nm. Theend portions of the source electrode 856 and the drain electrode 857 aretapered, whereby coverage with the insulating layer 828 formed later canbe improved and disconnection can be prevented. In this embodiment, as aconductive layer, a three-layer stacked film in which a 400-nm-thickaluminum layer is stacked over a 100-nm-thick titanium layer and a100-nm-thick titanium layer is stacked over the aluminum layer is used(see FIG. 5A).

The etching of the conductive layer may be performed by either a dryetching method or a wet etching method. For dry etching, an excitationmethod such as an inductively coupled plasma (ICP) etching method usinga gas containing chlorine or fluorine as an etching gas can be used.

Note that it is preferable that etching conditions be optimized so asnot to etch and divide the semiconductor layer when the conductive layeris etched. However, it is difficult to obtain etching conditions inwhich only the conductive layer is etched and the semiconductor layer isnot etched at all. In some cases, depending on the etching conditions,only part of the semiconductor layer is etched to be a semiconductorlayer having a groove portion (a recessed portion) when the conductivelayer is etched.

In addition, the conductive layer to be the source electrode 856 and thedrain electrode 857 (including a wiring layer formed from the same layeras the source electrode and the drain electrode) may be formed using aconductive metal oxide. As the conductive metal oxide, indium oxide(In₂O₃), tin oxide (SnO₂), zinc oxide (ZnO), indium oxide-tin oxidealloy (In₂O₃—SnO₂, which is abbreviated to ITO), indium oxide-zinc oxidealloy (In₂O₃—ZnO) or any of these metal oxide materials in which siliconoxide is contained can be used.

Next, the insulating layer 828 is formed over the semiconductor layer859, the source electrode 856, and the drain electrode 857. Before theinsulating layer 828 is formed, heat treatment may be performed at 300°C. in a nitrogen atmosphere for one hour. Further, before the insulatinglayer 828 is formed, plasma treatment using a gas such as dinitrogenmonoxide (N₂O), nitrogen (N₂), or argon (Ar) may be performed to removewater and the like attached to an exposed surface of the oxidesemiconductor layer. When the plasma treatment is performed, theinsulating layer 828 is formed without exposure to the air.

The insulating layer 828 can be formed to a thickness at least 1 nm by amethod by which an impurity such as water or hydrogen does not enter theinsulating layer 828, such as a sputtering method as appropriate. Whenhydrogen is contained in the insulating layer 828, entry of the hydrogento the oxide semiconductor layer or extraction of oxygen in the oxidesemiconductor layer by the hydrogen is caused, thereby making theresistance of the backchannel of the oxide semiconductor layer low (tohave an n-type conductivity), so that a parasitic channel might beformed. Therefore, it is important that a film formation method in whichhydrogen is not used be employed in order to form the insulating layer828 containing as little hydrogen as possible.

In this embodiment, a silicon oxide layer is formed to a thickness of300 nm as the insulating layer 828 by a sputtering method (see FIG. 5B).The silicon oxide layer can be deposited by a sputtering method in arare gas (typically, argon) atmosphere, an oxygen atmosphere, or a mixedatmosphere containing a rare gas and oxygen. As a target, a siliconoxide target or a silicon target can be used. For example, the siliconoxide layer can be formed using a silicon target by a sputtering methodin an atmosphere containing oxygen. As the insulating layer 828 that isformed in contact with the oxide semiconductor layer, an inorganicinsulating layer that does not contain impurities such as moisture, ahydrogen ion, and OH⁻ and blocks entry of these from the outside isused. Typically, a silicon oxide layer, a silicon oxynitride layer, analuminum oxide layer, an aluminum oxynitride layer, or the like is used.

Note that a protective insulating layer may be additionally formed overthe insulating layer 828. For example, a silicon nitride layer is formedby an RF sputtering method. Since an RF sputtering method has highproductivity, it is preferably used as a film formation method of theprotective insulating layer. As the protective insulating layer, aninorganic insulating layer that does not include an impurity such asmoisture and prevents entry of these from the outside, such as a siliconnitride layer or an aluminum nitride layer may be used.

Next, a second heat treatment is performed, preferably in an inert gasatmosphere or oxygen gas atmosphere (preferably at a temperature higherthan or equal to 200° C. and lower than or equal to 400° C., forexample, higher than or equal to 250° C. and lower than or equal to 350°C.). For example, the second heat treatment is performed in a nitrogenatmosphere at 250° C. for one hour. In the second heat treatment, partof the oxide semiconductor layer (a channel formation region) is heatedwhile being in contact with the insulating layer 828.

Through the above process, the first heat treatment is performed on theoxide semiconductor film so that an impurity such as hydrogen, moisture,a hydroxyl group, or hydride (also referred to as a hydrogen compound)is removed from the oxide semiconductor layer, and oxygen which is oneof main components of an oxide semiconductor and is reduced in the stepof removing impurities can be supplied by the second heat treatment.Accordingly, the oxide semiconductor layer is highly purified to be anelectrically i-type (intrinsic) semiconductor.

When a silicon oxide layer having a lot of defects is used as theinsulating layer 828, by the heat treatment performed after theformation of the silicon oxide layer, impurities such as hydrogen,moisture, hydroxyl, or hydride contained in the oxide semiconductorlayer can be diffused to the insulating layer 828 so that impurities inthe oxide semiconductor layer can be further reduced.

Note that in this embodiment, the second heat treatment is performedafter the insulating layer 828 is formed; however, the timing of thesecond heat treatment is not limited thereto. For example, the secondheat treatment may be performed after the source and drain electrodesare formed, or after the reflective electrode is formed.

Next, by a fifth photolithography step, part of the insulating layer 828that overlaps with the drain electrode 857 is selectively removed, sothat the contact hole 855 is formed (see FIG. 5C). Note that a resistmask may be formed by an inkjet method. Formation of the resist mask byan inkjet method needs no photomask; thus, manufacturing cost can bereduced. The etching of the insulating layer 828 here may be performedby either one or both of a dry etching method and a wet etching method.

Next, the etching-stop layer 829 is formed over the insulating layer828. The etching-stop layer 829 has a function of protecting theinsulating layer 828 from damage due to etching in the formation step ofthe structure 820 later.

As a material of the etching-stop layer 829, a conductive material, aninsulating material, or a semiconductor material can be used as long asit is a light-transmitting material that can withstand an etchingprocess in the formation process of the structure 820. As a material forthe etching-stop layer 829, indium oxide (In₂O₃), tin oxide (SnO₂), zincoxide (ZnO), an indium oxide-tin oxide alloy (In₂O₃—SnO₂, which isabbreviated to ITO), an indium oxide-zinc oxide alloy (In₂O₃—ZnO), analuminum oxide, or an aluminum oxynitride, or the like can be used. Theabove-described oxide semiconductors such as an In—Ga—Zn—O-based oxidesemiconductor can be used.

The thickness of the etching-stop layer 829 can be determined inaccordance with the height of the structure 820, and the selectivity inthe etching condition when the structure 820 is formed. The thickness ofthe etching-stop layer 829 is preferably greater than or equal to 10 nmand less than or equal to 100 nm. In this embodiment, a 50-nm-thickIn—Ga—Zn—O-based oxide semiconductor is used for the etching-stop layer829 (see FIG. 6A).

Next, an insulating layer 833 to form the structure 820 is formed overthe etching-stop layer 829. As a material for the insulating layer 833,a light-transmitting material such as silicon oxide (SiO_(x)), siliconnitride (SiN_(x)), silicon oxynitride (SiNO), an acrylic resin, apolyimide resin, a siloxane resin, phosphosilicate glass (PSG), orborophosphosilicate glass (BPSG) can be used.

The height of the structure 820 is determined by the thickness of theinsulating layer 833. The thickness of the insulating layer 833 isgreater than or equal to 1 μm, preferably greater than or equal to 3 μm,further preferably greater than or equal to 5 μm. As the structure 820is higher, the difference between the area of the backlight entrance 842and the area of the backlight exit 841 becomes larger; therefore, theamount of transmitted light from the opening 826 can be increasedwithout increasing the luminance of the backlight. In this embodiment, a4-μm-thick polyimide resin is formed as an insulating layer to form thestructure 820 (see FIG. 6B).

Next, by a sixth photolithography step, a resist mask 834 having atapered shape is formed using a photomask over the insulating layer 833(see FIG. 6B). Note that the tapered resist mask can be formed by aknown method.

Then, the insulating layer 833 is selectively etched by dry etching, sothat the structure 820 is formed. Dry etching is performed under thecondition that the selectivity between the resist mask 834 and theinsulating layer 833 is low, and the insulating layer 833 is etchedwhile the resist mask 834 is etched (see FIG. 7A). Accordingly, theinsulating layer 833 is etched while the resist mask 834 is downsized,so that the structure 820 that reflects the shape of the resist mask 834can be formed. In this embodiment, although a mixed gas of carbontetrafluoride (CF₄) and oxygen is used as an etching gas, there is nolimitation as long as it is a gas that can be used under the conditionthat the selectivity between the resist mask 834 and the insulatinglayer 833 is low.

Note that the angle of the tapered shape, that is θT, of the structure820 can be determined by a tapered shape of the resist mask 834 and theselectivity between the resist mask 834 and the insulating layer 833 inthe etching.

Although the etching-stop layer 829 is exposed when unnecessary portionsof the insulating layer 833 are removed, the etching-stop layer 829 ishardly etched by dry etching using a mixed gas of carbon tetrafluoride(CF₄) and oxygen because the etching-stop layer 829 used in thisembodiment is an In—Ga—Zn—O-based oxide semiconductor.

When the etching-stop layer 829 is not formed, damage to the insulatinglayer 828 in the bottom layer is caused by the dry etching, which leadsto a reduction in yield or variation in characteristics. In some cases,the insulating layer 828 is removed, which has a fatal impact on thesemiconductor layer 859. In particular, the tendency is increased as theinsulating layer 833 is thicker in order to heighten the structure 820.With the use of the etching-stop layer 829, productivity of asemiconductor device is increased, and a semiconductor device with highreproducibility of display and less variation in characteristics can beobtained.

After the dry etching is performed, the resist mask 834 is removed, sothat the structure 820 is completed (see FIG. 7B).

Next, the etching-stop layer 829 that is exposed and that is not underthe structure 820 is removed by etching. The etching here may beperformed by either dry etching or wet etching as long as the structure820 and the insulating layer 828 are not influenced very much; however,wet etching is preferable because damage to the structure 820 and theinsulating layer 828 is hardly caused. In this embodiment, theetching-stop layer 829 is etched using ITO-07N (produced by KANTOCHEMICAL CO., INC.) (see FIG. 7C). Note that the exposed etching-stoplayer 829 is not necessarily removed unless other steps and structureare adversely affected.

Next, a metal layer is formed over the structure 820 and the insulatinglayer 828, so that the reflective layer 821 is formed by the seventhphotolithography step. Note that a resist mask may be formed by aninkjet method. Formation of the resist mask by an inkjet method needs nophotomask; thus, manufacturing cost can be reduced.

The metal layer can be formed using a material with high reflectance ofvisible light such as aluminum (Al) or silver (Ag), or an alloyincluding any of these. A metal material such as molybdenum, titanium,tantalum, or tungsten may be stacked over the material with highreflectance. In this embodiment, a reflective layer 821 a and areflective layer 821 b are stacked as the reflective layer 821. Thereflective layer 821 a and the reflective layer 821 b are formed usingaluminum and titanium, respectively. With such a structure, generationof a hillock or a whisker in an aluminum layer can be prevented, areduction in reflectance and an increase in electric resistance due tomigration can be prevented.

The etching of the metal layer may be performed by either a dry etchingmethod or a wet etching method. For dry etching, an excitation methodsuch as an inductively coupled plasma (ICP) etching method using a gascontaining chlorine or fluorine as an etching gas can be used.

In addition, part of the reflective layer 821 can be used as a wiring.In this embodiment, the structure in which the reflective layer 821 andthe drain electrode 857 is connected through the contact hole 855 isshown (see FIG. 8A).

Next, part of the reflective layer 821 that is contact with thebacklight exit 841 of the structure 820 is removed. First, the resistmask 835 is formed over the reflective layer 821 (see FIG. 8B).

After that, the resist mask 835 is etched by dry etching until thereflective layer 821 over the backlight exit 841 of the structure 820 isexposed (see FIG. 8C). As the etching gas here, oxygen or a gascontaining oxygen may be used.

Next, the exposed portion of the reflective layer 821 are removed byetching (see FIG. 9A). Either dry etching or wet etching may be employedfor etching; however, dry etching is preferable because etching rate canbe easily controlled. For dry etching, an excitation method such as aninductively coupled plasma (ICP) etching method using a gas containingchlorine or fluorine as an etching gas can be used.

After that, the remaining resist mask 835 is removed, so that thestructure in which the reflective layer 821 is in contact with the sidesurfaces (surfaces other than the backlight exit 841 and the backlightentrance 842) of the structure 820 can be provided (see FIG. 9B).

Next, the transparent electrode 823 is formed over the structure 820 byan eighth photolithography step. As the transparent electrode 823,indium oxide (In₂O₃), tin oxide (SnO₂), zinc oxide (ZnO), indiumoxide-tin oxide alloy (In₂O₃—SnO₂, which is abbreviated to ITO), indiumoxide-zinc oxide alloy (In₂O₃—ZnO), or any of these metal oxidematerials in which silicon oxide is contained can be used (see FIG. 9C).

In this embodiment, since the transparent electrode 823 is formed usingthe photo mask that is used in the seventh photolithography step, thetransparent electrode 823 is stacked over the reflective layer 821 andis connected to the drain electrode 857 through the reflective layer821. Note that when different masks are used between the seventhphotolithography step and the eighth photolithography step, a structuremay be provided in which the reflective layer 821 is not formed in thecontact hole 855, so that the transparent electrode 823 and the drainelectrode 857 are directly connected to each other.

Next, the organic resin layer 822 having an uneven curving top surfaceis formed over the transparent electrode 823. For the organic resinlayer 822, a photosensitive organic resin such as acrylic, polyimide,benzocyclobutene, phenol, or polystyrene may be used. The organic resinlayer 822 is partly exposed to light by a ninth photolithography withthe use of the photosensitive organic resin, so that the organic resinlayer 822 can have an uneven curving top surface without using a resistmask (see FIG. 10A).

The shape and depth of the uneven curving surface can be adjustedarbitrarily by adjusting intensity and time of the light exposure, andthe like. By forming the photo mask by a method of forming a gray-tonemask, the uneven curving surface can be formed over the organic resinlayer 822 and the contact hole 865 can be formed in the organic resinlayer 822 at the same time without changing intensity and time of thelight exposure (see FIG. 1C).

Next, a conductive layer is formed over the organic resin layer 822, sothat the reflective electrode 825 having the opening 826 is formed by atenth photolithography step. Note that a resist mask may be formed by aninkjet method. Formation of the resist mask by an inkjet method needs nophotomask; thus, manufacturing cost can be reduced.

The conductive layer can be formed using a material with highreflectance of visible light such as aluminum (Al) or silver (Ag), or analloy including any of these. A metal material such as molybdenum,titanium, tantalum, or tungsten may be provided between the conductivelayer and the organic resin layer 822. In this embodiment, a reflectiveelectrode 825 a and a reflective electrode 825 b are stacked as thereflective electrode 825. The reflective electrode 825 a and thereflective electrode 825 b are formed using aluminum and titanium,respectively (see FIG. 10B). With such a structure, generation of ahillock or a whisker in an aluminum layer can be prevented, a reductionin reflectance and an increase in electric resistance due to migrationcan be prevented.

The etching of the conductive layer may be performed by either a dryetching method or a wet etching method. For dry etching, an excitationmethod such as an inductively coupled plasma (ICP) etching method usinga gas containing chlorine or fluorine as an etching gas can be used.

In this embodiment, the conductive layer is etched by a dry etchingmethod, so that the reflective electrode 825 having the opening 826 isformed. In addition, the organic resin layer 822 in the opening 826 isalso etched in the etching of the conductive layer, so that thetransparent electrode 823 is exposed.

By reflecting the curving surface with an uneven shape of the organicresin layer 822 on the reflective electrode 825, the area of thereflective region can be increased, and reflection of an object fromoutside is reduced so that visibility of the displayed image can beimproved.

Through the above steps, the semiconductor device disclosed in thisembodiment can be fabricated.

In a transistor including an oxide semiconductor layer, the currentvalue in an off state (the off-state current value) can be small.Therefore, an electric signal such as image data can be held for alonger period and a writing interval can be set longer. Consequently,frequency of refresh operation can be reduced, which leads to an effectof suppressing power consumption.

In a transistor including an oxide semiconductor, relatively highfield-effect mobility can be obtained, whereby high-speed operation ispossible. Accordingly, by using the transistor in a pixel portion of aliquid crystal display device, a high-quality image can be provided.Since the transistors can be separately formed over one substrate in acircuit portion and a pixel portion, the number of components can bereduced in the liquid crystal display device.

By using this embodiment, a semi-transmissive liquid crystal displaydevice with bright and high-quality display can be obtained withoutincreasing power consumption.

Embodiment 2

In this embodiment, an example of a transistor that can be applied to aliquid crystal display device disclosed in this specification isdescribed. There is no particular limitation on the structure of thetransistor; for example, a staggered type transistor or a planar typetransistor having a top-gate structure or a bottom-gate structure can beemployed. Further, the transistor may have a single gate structureincluding one channel formation region, a double gate structureincluding two channel formation regions, or a triple gate structureincluding three channel formation regions. Alternatively, the transistormay have a dual-gate structure including two gate electrodes positionedabove and below a channel region with gate insulating layers providedtherebetween. Note that examples of cross-sectional structures oftransistors illustrated FIGS. 11A to 11D are described below.Transistors illustrated in FIGS. 11A to 11D are transistors each usingan oxide semiconductor as a semiconductor. An advantage of using anoxide semiconductor is that high mobility and low off-state current canbe obtained in a relatively easy and low-temperature process; however,it is needless to say that another semiconductor may be used.

The composition material of the transistor disclosed in Embodiment 1 canbe used as that of the transistors disclosed in this embodiment.

A transistor 410 illustrated in FIG. 11A is one of bottom-gatetransistors, and is also one of inverted staggered transistors.

The transistor 410 includes, over a light-transmitting substrate 400, agate electrode 401, a gate insulating layer 402, an oxide semiconductorlayer 403, a source electrode 405 a, and a drain electrode 405 b.Further, an insulating layer 407 stacked over the oxide semiconductorlayer 403 is provided so as to cover the transistor 410. A protectiveinsulating layer 409 is formed over the insulating layer 407.

A transistor 420 illustrated in FIG. 11B is one of bottom-gatetransistors called channel-protective (channel-stop) transistors and isalso one of inverted staggered transistors.

The transistor 420 includes, over the light-transmitting substrate 400,the gate electrode 401, the gate insulating layer 402, the oxidesemiconductor layer 403, an insulating layer 427 functioning as achannel protective layer which covers a channel formation region of theoxide semiconductor layer 403, the source electrode 405 a, and the drainelectrode 405 b. Further, the protective insulating layer 409 is formedso as to cover the transistor 420.

A transistor 430 illustrated in FIG. 11C is one of bottom-gatetransistors. The transistor 430 includes, over the light-transmittingsubstrate 400, the gate electrode 401, the gate insulating layer 402,the source electrode 405 a, the drain electrode 405 b, and the oxidesemiconductor layer 403. Further, the insulating layer 407 being incontact with the oxide semiconductor layer 403 is provided so as tocover the transistor 430. A protective insulating layer 409 is formedover the insulating layer 407.

In the transistor 430, the gate insulating layer 402 is provided on andin contact with the substrate 400 and the gate electrode 401, and thesource electrode 405 a and the drain electrode 405 b are provided on andin contact with the gate insulating layer 402. Further, the oxidesemiconductor layer 403 is provided over the gate insulating layer 402,the source electrode 405 a, and the drain electrode 405 b.

A transistor 440 illustrated in FIG. 11D is one of top-gate transistors.The transistor 440 includes, over the light-transmitting substrate 400,an insulating layer 437, the oxide semiconductor layer 403, the sourceelectrode 405 a, the drain electrode 405 b, the gate insulating layer402, and the gate electrode 401. A wiring layer 436 a a wiring layer 436b are provided to be in contact with and electrically connected to thesource electrode 405 a and the drain electrode 405 b, respectively.

In the transistors 410, 420, 430, and 440 each including the oxidesemiconductor layer 403, the current value in an off state (an off-statecurrent value) can be reduced. Therefore, electric signal of image dataand the like can be held for a longer period, so that a writing intervalcan be set long. Accordingly, frequency of refresh operation can bereduced, which leads to an effect of suppressing power consumption.

Further, in the transistors 410, 420, 430, and 440 each including theoxide semiconductor layer 403, relatively high field-effect mobility canbe obtained, whereby high-speed operation is possible. Therefore, byusing any of the transistors in a pixel portion of a liquid crystaldisplay device, high-quality image can be provided. Since thetransistors can be separately formed over one substrate in a circuitportion and a pixel portion, the number of components can be reduced inthe liquid crystal display device.

Thus, a high-performance liquid crystal display device can be providedby using a transistor including an oxide semiconductor layer.

Embodiment 3

In this embodiment, a structural example of a semi-transmissive liquidcrystal display module will be described. The semi-transmissive liquidcrystal display module described in this embodiment displays images inmono-color display when used in a reflective mode, and displays infull-color display when used in a transparent mode.

FIG. 12 illustrates a structure of a liquid crystal display module 190.The liquid crystal display module 190 includes the backlight portion130, the display panel 120 in which liquid crystal elements are arrangedin matrix, and a polarizing plate 125 a and a polarizing plate 125 bwhich are provided with the display panel 120 positioned therebetween.The backlight portion 130 is a surface-emitting backlight portion thatemits uniform white light. For example, the backlight portion 130 mayinclude a white LED 133 placed in an end portion of a light guide plateand a diffusing plate 134 provided between the light guide plate and thedisplay panel 120. In addition, a flexible printed circuit (FPC) 140serving as an external input terminal is electrically connected to aterminal portion provided in the display panel 120.

Note that each of the liquid crystal elements provided in the displaypanel 120 has a structure similar to that of the pixel described inEmbodiment 1.

First, a method for displaying images in the reflective mode isdescribed. In FIG. 12, an arrow is used to illustrate schematically howexternal light 139 is transmitted through the liquid crystal elements inthe display panel 120 and reflected by a reflective electrode to theviewer side. The external light 139 is transmitted through a liquidcrystal layer, reflected by the reflective electrode, and transmittedthrough the liquid crystal layer again to be extracted. The intensity ofthe light that is transmitted through the liquid crystal elements ismodulated by an image signal. Therefore, a viewer can perceive an imageby reflected light of the external light 139.

Next, a method for displaying images in the transmissive mode isdescribed. In FIG. 12, arrows indicating light 135 of three colors (R,G, and B) are used to illustrate schematically how light from thebacklight portion 130 enters a back surface of the display panel 120 andis transmitted to the viewer side through a structure (an anisotropiclight-condensing means), a coloring layer, a transmissive electrode, andthe liquid crystal elements provided in the display panel. For example,in a part of the pixel overlapping with a red coloring layer functioningas a color filter, light from the backlight is condensed to the redcoloring layer by the structure (the anisotropic light-condensing means)provided in the display panel, and transmitted through the coloringlayer, the transmissive electrode, and the liquid crystal elements to beextracted as red light. The intensity of the light transmitted throughthe liquid crystal elements is modulated by an image signal; therefore,a viewer can perceive an image by the light 135 of three colors. Notethat since full-color display is employed, a circuit configuration isemployed in which three display elements of a red display element, agreen display element, and a blue display element are supplied withrespective video signals different from each other.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

Embodiment 4

In this embodiment, one mode of a liquid crystal display device of thepresent invention and a driving method thereof in which low powerconsumption can be achieved will be described with reference to FIG. 13,FIG. 14, FIG. 15, FIG. 16, FIG. 17, FIGS. 18A and 18B, and FIG. 19.

Components of a liquid crystal display device 100 described in thisembodiment are illustrated in a block diagram of FIG. 13. The liquidcrystal display device 100 includes an image processing circuit 110, apower supply 116, a display control circuit 113, and the display panel120. In the case of a transmissive liquid crystal display device or asemi-transmissive liquid crystal display device, the backlight portion130 is further provided as a light source.

To the liquid crystal display device 100, an image signal (an imagesignal Data) is supplied from an external device connected to the liquidcrystal display device. Note that power supply potential (high powersupply potential Vdd, low power supply potential Vss, and commonpotential Vcom) is supplied by turning on the power supply 116 of theliquid crystal display device and starting supplying power, and controlsignals (a start pulse SP and a clock signal CK) are supplied by thedisplay control circuit 113.

Note that the high power supply potential Vdd is a potential higher thana reference potential, and the low power supply potential Vss is apotential lower than or equal to the reference potential. Note that itis desirable that each of the high power supply potential Vdd and thelow power supply potential Vss be a potential such that a transistor canoperate. The high power supply potential Vdd and the low power supplypotential Vss are collectively referred to as a power supply voltage ora power supply potential in some cases.

The common potential Vcom may be any potential as long as it serves asreference with respect to the potential of an image signal supplied to apixel electrode. For example, the common potential Vcom may be a groundpotential.

Note that the image signal Data may be appropriately inverted inaccordance with dot inversion driving, source line inversion driving,gate line inversion driving, frame inversion driving, or the like to beinput to the liquid crystal display device 100. When the image signal isan analog signal, it may be converted to a digital signal through an A/Dconverter or the like to be supplied to the liquid crystal displaydevice 100.

In this embodiment, the common potential Vcom that is a fixed potentialis supplied from the power supply 116 to a common electrode 128 and oneelectrode of a capacitor 210 through the display control circuit 113.

The display control circuit 113 is a circuit which supplies a displaypanel image signal (Data), a control signal (specifically, a signal forcontrolling switching between supply and stop of the control signal suchas the start pulse SP and the clock signal CK), and the power supplypotential (the high power supply potential Vdd, the low power supplypotential Vss, and the common potential Vcom) to the display panel 120.

The image processing circuit 110 analyzes, calculates, and processes aninput image signal (image signal Data), and then outputs the processedimage signal together with a control signal to the display controlcircuit 113.

Specifically, the image processing circuit 110 analyzes the input imagesignal Data, determines whether the input image signal Data is for amoving image or a still image, and outputs a control signal includingthe determination result to the display control circuit 113. Further,the image processing circuit 110 captures a still image of one framefrom the image signal Data including a moving image or a still image andoutputs the captured image together with a control signal whichindicates that the captured image is a still image to the displaycontrol circuit 113. The image processing circuit 110 outputs the inputimage signal Data together with the above control signal to the displaycontrol circuit 113. Note that the above-described function is anexample of functions that the image processing circuit 110 has, and avariety of image processing functions may be selected and applied inaccordance with applications of the display device.

Note that since an image signal which is converted to a digital signalis easily calculated (e.g., detection of a difference between imagesignals), when an input image signal (image signal Data) is an analogsignal, an A/D converter or the like is provided in the image processingcircuit 110.

The display panel 120 has a structure in which a liquid crystal element215 is provided between a pair of substrates (a first substrate and asecond substrate). Specifically, the transparent electrode transmittingvisible light, a reflecting electrode reflecting visible light, and thestructure 820 (the anisotropic light-condensing means having acondensing direction X and a non-condensing direction Y) overlappingwith the transparent electrode which are described in Embodiment 1 areprovided on the same surface side of the first substrate. The firstsubstrate is provided with a driver circuit portion 121 and a pixelportion 122. The second substrate is provided with a common connectionportion (also referred to as a common contact) and the common electrode128 (also referred to as a counter electrode). The common connectionportion electrically connects the first substrate and the secondsubstrate. The common connection portion may be provided over the firstsubstrate.

In the pixel portion 122, a plurality of gate lines (scan lines) 124 anda plurality of source lines (signal lines) 125 are provided. A pluralityof pixels 123 is provided in matrix so as to be surrounded by the gatelines 124 and the source lines 125. In the display panel described inthis embodiment, the gate line 124 and the source line 125 are extendedfrom a gate line side driver circuit 121A and a source line side drivercircuit 121B, respectively.

In addition, the pixel 123 includes a transistor 214 as a switchingelement, and the capacitor 210 and the liquid crystal element 215 whichare connected to the transistor 214.

The liquid crystal element 215 is an element that controls transmissionand non-transmission of light by an optical modulation action of liquidcrystal. The optical modulation action of liquid crystal is controlledby an electric field applied to the liquid crystal. The direction of theelectric field applied to the liquid crystal varies according to aliquid crystal material, a driving method, and an electrode structureand can be selected as appropriate. For example, when a driving methodin which an electric field is applied in a thickness direction (aso-called vertical direction) of liquid crystal is used, the pixelelectrode and the common electrode may be provided over the firstsubstrate and the second substrate, respectively, so as to sandwich theliquid crystal. When a driving method in which an electric field isapplied in an in-plane direction of a substrate (a so-called horizontaldirection) to liquid crystal is used, the pixel electrode and the commonelectrode may be provided over the same substrate with respect to theliquid crystal. The pixel electrode and the common electrode may have avariety of opening patterns. In this embodiment, there is no particularlimitation on a liquid crystal material, a driving method, and anelectrode structure as long as an element controls transmission andnon-transmission of light by the optical modulation action.

In the transistor 214, one of the plurality of gate lines 124 providedin the pixel portion 122 is connected to the gate electrode, one of asource electrode and a drain electrode is connected to one of theplurality of source lines 125, and the other of the source electrode andthe drain electrode is connected to one of the electrodes of thecapacitor 210 and one of the electrodes of the liquid crystal element215 (pixel electrode).

With such a structure, the capacitor 210 can hold voltage applied to theliquid crystal element 215. The electrode of the capacitor 210 may beconnected to a capacitor line additionally provided.

As the transistor 214, a transistor whose off-state current is reducedis preferably used. When the off-state current is reduced, thetransistor 214 in an off-state can stably hold charge in the liquidcrystal element 215 and the capacitor 210. Further, when the transistor214 whose off-state current is sufficiently reduced is used, the pixel123 can be formed without providing the capacitor 210.

The driver circuit portion 121 includes the gate line side drivercircuit 121A and the source line side driver circuit 121B. The gate lineside driver circuit 121A and the source line side driver circuit 121Bare driver circuits for driving the pixel portion 122 that includes aplurality of pixels and each include a shift register circuit (alsoreferred to as a shift register).

Note that the gate line side driver circuit 121A and the source lineside driver circuit 121B may be formed over the same substrate as thepixel portion 122 or over another substrate.

Note that the high power supply potential Vdd, the low power supplypotential Vss, the start pulse SP, the clock signal CK, and the imagesignal Data which are controlled by the display control circuit 113 aresupplied to the driver circuit portion 121.

A terminal portion 126 is an input terminal which supplies apredetermined signal output from the display control circuit 113 (suchas the high power supply potential Vdd, the low power supply potentialVss, the start pulse SP, the clock signal CK, the image signal Data, andthe common potential Vcom) and the like, to the driver circuit portion121.

The common electrode 128 is electrically connected to a common potentialline for supplying the common potential Vcom that is controlled by thedisplay control circuit 113, in the common connection portion.

As a specific example of the common connection portion, the commonelectrode 128 and the common potential line can be electricallyconnected with a conductive particle in which an insulating sphere iscovered with a thin metal film provided therebetween. Note that two ormore common connection portions may be provided in the display panel120.

The liquid crystal display device may include a photometric circuit. Theliquid crystal display device provided with the photometric circuit candetect the brightness of the environment where the liquid crystaldisplay device is put. Thus, the display control circuit 113 connectedto the photometric circuit can control a driving method of a lightsource such as a backlight and a sidelight in accordance with a signalinput from the photometric circuit.

The backlight portion 130 includes a backlight control circuit 131 and abacklight 132. The backlight 132 may be selected and combined inaccordance with applications of the liquid crystal display device 100,and a light-emitting diode (LED) or the like can be used. For example, awhite light-emitting element (e.g., LED) can be provided in thebacklight 132. A backlight signal for controlling the backlight and thepower supply potential are supplied from the display control circuit 113to the backlight control circuit 131.

Note that an optical film (such as a polarizing film, a retardationfilm, or an anti-reflection film) can also be used in combination. Alight source such as a backlight that is used in a semi-transmissiveliquid crystal display device may be selected and combined in accordancewith the use of the liquid crystal display device 100, and a coldcathode tube, a light-emitting diode (LED), or the like can be used.Further, a surface light source may be formed using a plurality of LEDlight sources, a plurality of electroluminescent (EL) light sources, orthe like. As the surface light source, three or more kinds of LEDs maybe used and an LED emitting white light may be used. Note that the colorfilter is not always provided when light-emitting diodes of RGB or thelike are arranged in a backlight and a successive additive color mixingmethod (a field sequential method) in which color display is performedby time division is employed.

Next, a detail and a driving method of the liquid crystal display deviceillustrated in FIG. 13 will be described with reference to FIG. 14, FIG.15, FIG. 16, FIG. 17, FIGS. 18A and 18B, and FIG. 19. The driving methodof the liquid crystal display device described in this embodiment is adisplay method in which the frequency of rewriting in the display panelvaries in accordance with properties of an image to be displayed.Specifically, when image signals in successive frames are different fromeach other (i.e., a moving image is displayed), a display mode in whichan image signal is written to every frame is employed. On the otherhand, when image signals in successive frames are the same (i.e., astill image is displayed), the following display mode is employed: imagesignals are prevented from being written or the writing frequency isextremely reduced in a period during which the same image is beingdisplayed; the voltage applied to the liquid crystal element is held bysetting potentials of the pixel electrode and the common electrode forapplying a voltage to the display element in a floating state; andaccordingly a still image is displayed without an additional supply ofpotential.

The liquid crystal display device displays a moving image and a stillimage in combination on its screen. By switching of a plurality ofdifferent images which is time-divided into a plurality of frames athigh speed, the images are recognized as a moving image by human eyes.Specifically, by switching of images at least 60 times (60 frames) persecond, the images are recognized as a moving image with less flicker byhuman eyes. In contrast, unlike a moving image and a partial movingimage, a still image is an image which does not change in successiveframe periods, for example, in an n-th frame and an (n+1)th frame, evenwhen a plurality of images which are time-divided into a plurality offrame periods are switched at high speed.

The liquid crystal display device according to the present invention canbe operated in different display modes, a moving-image display mode anda still-image display mode, in the case of displaying a moving image anddisplaying a still image, respectively. In this specification, an imagedisplayed in the case of displaying a still image is also referred to asa still image.

Next, components of the liquid crystal display device 100 of thisembodiment are described with reference to a block diagram of FIG. 14.The liquid crystal display device 100 is an example of asemi-transmissive liquid crystal display device in which a liquidcrystal layer controls transmission and non-transmission of light in apixel so that display is performed, and includes the image processingcircuit 110, the power supply 116, the display panel 120, and thebacklight portion 130.

To the liquid crystal display device 100, an image signal (an imagesignal Data) is supplied from an external device connected to the liquidcrystal display device. Note that power supply potential (high powersupply potential Vdd, low power supply potential Vss, and commonpotential Vcom) is supplied by turning on the power supply 116 of theliquid crystal display device and starting supplying power, and controlsignals (a start pulse SP and a clock signal CK) are supplied by thedisplay control circuit 113. The high power supply potential Vdd issupplied to a Vdd line (not shown) included in the driver circuitportion 121, and the low power supply potential Vss is supplied to a Vssline (not shown) included in the driver circuit portion 121.

Next, a structure of the image processing circuit 110 and a procedure inwhich the image processing circuit 110 processes signals are describedwith reference to FIG. 14 as an example. Note that the image processingcircuit 110 illustrated in FIG. 14 is just one mode of this embodimentand this embodiment is not limited to this structure.

The image processing circuit 110 illustrated in FIG. 14 analyzes imagesignals which are successively input and determines whether the inputimage signal is for a moving image or a still image. When the input ofimage signals (image signal Data) is switched from an input of a movingimage signal to an input of a still image signal, the image processingcircuit 110 captures a still image and outputs the captured imagetogether with a control signal which indicates that the captured imageis a still image to the display control circuit 113. When the input ofimage signals (image signal Data) is switched from an input of a stillimage signal to an input of a moving image signal, the image processingcircuit 110 outputs an image signal including a moving image togetherwith a control signal which indicates that the image signal is a movingimage to the display control circuit 113.

The image processing circuit 110 includes a memory circuit 111, acomparison circuit 112, the display control circuit 113, and a selectioncircuit 115. The image processing circuit 110 generates a display panelimage signal and a backlight signal from the digital image signal Datathat is input. The display panel image signal is an image signal thatcontrols the display panel 120. The backlight signal is a signal thatcontrols the backlight portion 130. The image processing circuit 110outputs a signal that controls the common electrode 128 to a switchingelement 127.

The memory circuit 111 includes a plurality of frame memories forstoring image signals for a plurality of frames. The number of framememories included in the memory circuit 111 is not particularly limitedas long as the image signals for a plurality of frames can be stored.Note that the frame memory may be formed using a memory element such asdynamic random access memory (DRAM) or static random access memory(SRAM).

Note that the number of frame memories is not particularly limited aslong as an image signal can be stored for each frame period. Inaddition, the image signals stored in the frame memories are selectivelyread out by the comparison circuit 112 and the display control circuit113. A frame memory 111 b in the diagram illustrates a memory region forone frame conceptually.

The comparison circuit 112 is a circuit that selectively reads out imagesignals in successive frame periods stored in the memory circuit 111,compares the image signals in the successive frame periods in eachpixel, and detects a difference thereof.

In this embodiment, depending on whether a difference of image signalsbetween frames is detected or not, operations in the display controlcircuit 113 and the selection circuit 115 are determined. When adifference is detected in any of the pixels between frames by thecomparison circuit 112 (when there is a difference), the comparisoncircuit 112 determines that the image signal is not a signal fordisplaying a still image and that the successive frame periods duringwhich a difference is detected is a period during which a moving imageis to be displayed.

On the other hand, when a difference is not detected in any of thepixels by the comparison between image signals in the comparison circuit112 (when there is no difference), the successive frame periods duringwhich the difference is not detected is determined as a period duringwhich a still image is to be displayed. In other words, by detection ofthe differences in the comparison circuit 112, it is determined whetherthe image signals in successive frame periods are image signals fordisplaying moving images or image signals for displaying still images.

Note that the criterion of determining that there is a difference by thecomparison may be set such that the difference is recognized when thedifference exceeds a certain value. The comparison circuit 112 may beset to determine detection of a difference by the absolute value of thedifference.

Although, in this embodiment, the structure in which whether an image isa moving image or a still image is determined by detection of thedifference between the image signals in successive frame periods by thecomparison circuit 112 provided inside the liquid crystal display device100 is described, a structure in which a signal indicating whether theimage is a still image or a moving image is supplied from the outsidemay be used.

The selection circuit 115 employs a structure in which a plurality ofswitches formed of transistors are provided, for example. When thecomparison circuit 112 detects a difference in successive frame periods,that is, the image is a moving image, the selection circuit 115 selectsan image signal of the moving image from the frame memories in thememory circuit 111 and outputs the image signal to the display controlcircuit 113.

Note that when the comparison circuit 112 does not detect a differencein the successive frame periods, that is, the image is a still image,the selection circuit 115 does not output the image signal to thedisplay control circuit 113 from the frame memories in the memorycircuit 111. With the structure in which an image signal is not outputto the display control circuit 113 from the frame memory, powerconsumption of the liquid crystal display device can be reduced.

Note that in the liquid crystal display device of this embodiment, amode of operation performed when the comparison circuit 112 determinesan image as a still image is described as a still-image display mode,and a mode of operation performed when the comparison circuit 112determines an image as a moving image is described as a moving-imagedisplay mode.

The display control circuit 113 is a circuit which supplies an imagesignal selected by the selection circuit 115, a control signal(specifically, a signal for controlling switching between supply andstop of the control signal such as the start pulse SP and the clocksignal CK), and the power supply potential (the high power supplypotential Vdd, the low power supply potential Vss, and the commonpotential Vcom) to the display panel 120 and which supplies a backlightcontrol signal (specifically, a signal for the backlight control circuit131 to control on and off of the backlight) to the backlight portion130.

Note that the image processing circuit described in this embodiment asan example may have a display-mode switching function. The display-modeswitching function is a function of switching between a moving-imagedisplay mode and a still-image display mode in such a manner that a userof the liquid crystal display device selects an operation mode of theliquid crystal display device by hand or using an external connectiondevice.

The selection circuit 115 can output the image signal to the displaycontrol circuit 113 in accordance with a signal input from adisplay-mode switching circuit.

For example, when a mode-switching signal is input to the selectioncircuit 115 from the display-mode switching circuit while operation isperformed in a still-image display mode, even when the comparisoncircuit 112 does not detect the difference of the image signals insuccessive frame periods, the selection circuit 115 can be operated in amode in which image signals which are input are sequentially output tothe display control circuit 113, that is, in a moving-image displaymode. When a mode-switching signal is input to the selection circuit 115from the display-mode switching circuit while operation is performed ina moving-image display mode, even when the comparison circuit 112detects the difference of the image signal in successive frame periods,the selection circuit 115 can be operated in a mode in which only animage signal of one selected frame is output, that is, in a still-imagedisplay mode. As a result, in the liquid crystal display device of thisembodiment, one frame among moving images is displayed as a still image.

Further, when the liquid crystal display device includes a photometriccircuit, when the photometric circuit detects brightness and finds thatthe liquid crystal display device is used in a dim environment, thedisplay control circuit 113 controls light of the backlight 132 to havehigher intensity, so that favorable visibility of a display screen issecured. In contrast, when it is found that the liquid crystal displaydevice is used under extremely bright external light (e.g., under directsunlight outdoors), the display control circuit 113 controls light ofthe backlight 132 to have lower intensity, so that power consumed by thebacklight 132 is reduced.

In this embodiment, the display panel 120 includes the switching element127 besides the pixel portion 122. In this embodiment, the display panel120 includes the first substrate and the second substrate. The firstsubstrate is provided with the driver circuit portion 121, the pixelportion 122, and the switching element 127.

The pixel 123 includes the transistor 214 as a switching element, andthe capacitor 210 and the liquid crystal element 215 which are connectedto the transistor 214 (see FIG. 15).

A transistor whose off-state current is reduced is preferably used forthe transistor 214. When the transistor 214 is off, charges stored inthe capacitor 210 and the liquid crystal element 215 which are connectedto the transistor 214 whose off-state current is reduced are less likelyto leak through the transistor 214, and a state where data is writtenbefore the transistor 214 is turned off can be held for a long time.

In this embodiment, liquid crystal is controlled by a vertical electricfield that is generated by the pixel electrode provided over the firstsubstrate and the common electrode provided over the second substratewhich faces the first substrate.

As an example of liquid crystal applied to a liquid crystal element, thefollowing can be given: a nematic liquid crystal, a cholesteric liquidcrystal, a smectic liquid crystal, a discotic liquid crystal, athermotropic liquid crystal, a lyotropic liquid crystal, a low-molecularliquid crystal, a high-molecular liquid crystal, a polymer dispersedliquid crystal (PDLC), a ferroelectric liquid crystal, ananti-ferroelectric liquid crystal, a main-chain liquid crystal, aside-chain high-molecular liquid crystal, a banana-shaped liquidcrystal, and the like.

Alternatively, liquid crystal exhibiting a blue phase for which analignment layer is unnecessary may be used. A blue phase is one ofliquid crystal phases, which is generated just before a cholestericphase changes into an isotropic phase while temperature of cholestericliquid crystal is increased. Since the blue phase is only generatedwithin a narrow range of temperature, a chiral agent or an ultravioletcurable resin is added so that the temperature range is improved. Theliquid crystal composition which includes a liquid crystal showing ablue phase and a chiral agent is preferable because it has a smallresponse time of 10 μsec to 100 μsec, has optical isotropy, which makesthe alignment process unneeded, and has a small viewing angledependence.

In addition, as a driving method of liquid crystal, the following can beused: a TN (twisted nematic) mode, an STN (super twisted nematic) mode,an OCB (optically compensated birefringence) mode, an ECB (electricallycontrolled birefringence) mode, an FLC (ferroelectric liquid crystal)mode, an AFLC (anti-ferroelectric liquid crystal) mode, a PDLC (polymerdispersed liquid crystal) mode, a PNLC (polymer network liquid crystal)mode, a guest-host mode, and the like.

The switching element 127 supplies a common potential Vcom to the commonelectrode 128 in accordance with a control signal output from thedisplay control circuit 113. As the switching element 127, a transistorcan be used. A gate electrode of the transistor and one of a sourceelectrode and a drain electrode of the transistor may be connected tothe display control circuit 113 so that the common potential Vcom issupplied from the display control circuit 113 to the one of the sourceelectrode and the drain electrode of the transistor through the terminalportion 126. The other of the source electrode and the drain electrodeof the transistor may be connected to the common electrode 128. Notethat the switching element 127 may be formed over the same substrate asthe driver circuit portion 121 or the pixel portion 122. Alternatively,the switching element 127 may be formed over another substrate.

A transistor whose off-state current is reduced is used as the switchingelement 127, whereby a reduction over time in the voltage applied toboth terminals of the liquid crystal element 215 can be suppressed.

One of the source electrode and the drain electrode of the switchingelement 127 is electrically connected to the common electrode 128through the common connection portion. Note that the common electrode128 serves as one electrode of the capacitor 210 and one electrode ofthe liquid crystal element 215.

The other of the source electrode and the drain electrode of theswitching element 127 is connected to a terminal 126B. A gate electrodeof the switching element 127 is connected to a terminal 126A.

A pixel structure in an equivalent circuit diagram of a liquid crystaldisplay device illustrated in FIG. 16 is different from the pixelstructure illustrated in FIG. 15. Note that description of the samestructure as the equivalent circuit diagram of the liquid crystaldisplay device in FIG. 15 is omitted to avoid repeated description.

A pixel 136 includes a first pixel 136 a and a second pixel 136 b.

The first pixel 136 a includes a first transistor 224 a, a capacitor 210connected to the first transistor 224 a, and a liquid crystal element215. The liquid crystal element 215 is provided between a pixelelectrode connected to the first transistor 224 a and a counterelectrode facing the pixel electrode. The pixel electrode connected tothe first transistor 224 a reflects incident light through a liquidcrystal layer. The first transistor 224 a is connected to a gate lineside driver circuit 121A through a gate line 124 and connected to asource line side driver circuit 121B through a source line 125.

The second pixel 136 b includes a second transistor 224 b and acapacitor element 220 and a liquid crystal element 225 connected to thesecond transistor 224 b. The liquid crystal element 225 is providedbetween a pixel electrode connected to the second transistor 224 b and acounter electrode facing the pixel electrode. The pixel electrodeconnected to the second transistor 224 b has a light-transmittingproperty. The second transistor 224 b is connected to a gate line sidedriver circuit 121C through a gate line 137 and connected to a sourceline side driver circuit 121D through a source line 138.

The gate line side driver circuit 121A, the source line side drivercircuit 121B, the gate line side driver circuit 121C, and the sourceline side driver circuit 121D are connected to the display controlcircuit 113. The display control circuit 113 determines a signal line towhich the display control circuit 113 outputs an image signal.

Specifically, when the comparison circuit 112 determines that an imagesignal is a still image, the display control circuit 113 outputs theimage signal to the first pixel 136 a. When the comparison circuit 112determines that the image signal is a moving image, the display controlcircuit 113 outputs the image signal to the second pixel 136 b.

The display device described in this embodiment may include aphotometric circuit. The display device provided with the photometriccircuit can detect the brightness of the environment where the displaydevice is put. Therefore, the display control circuit 113 connected tothe photometric circuit can change a driving method of the display panel120 in accordance with a signal input from the photometric circuit.

For example, when the photometric circuit detects the display devicedescribed in this embodiment that is used in a dim environment, thedisplay control circuit 113 outputs an image signal to the second pixel136 b and the backlight 132 is turned on even when the comparisoncircuit 112 determines that the image signal is a still image. Since thesecond pixel 136 b includes the light-transmitting pixel electrode, astill image with high visibility can be provided using the backlight.

For example, when the photometric circuit detects the display devicedescribed in this embodiment which is used under extremely brightexternal light (e.g., under direct sunlight outdoors), the displaycontrol circuit 113 outputs an image signal to the first pixel 136 aeven when the comparison circuit 112 determines that the image signal isa moving image. Since the first pixel 136 a includes a pixel electrodewhich reflects incident light through the liquid crystal layer, a stillimage with high visibility can be provided even under extremely brightexternal light.

In other words, when the comparison circuit 112 determines that an imagesignals is a still image, the display control circuit 113 can output theimage signal to the second pixel 136 b, and when the comparison circuit112 determines that the image signal is a moving image, the displaycontrol circuit 113 can output the image signal to the first pixel 136a.

According to the structure of this embodiment, there is a choice whetherthe backlight is used or not depending on the usage environment, whichis convenient. In addition, power consumption is extremely low when astill image is displayed without use of the backlight.

Next, the state of signals supplied to the pixels will be described withreference to an equivalent circuit diagram of the liquid crystal displaydevice illustrated in FIG. 15 and a timing chart illustrated in FIG. 17.

In FIG. 17, a clock signal GCK and a start pulse GSP supplied from thedisplay control circuit 113 to the gate line side driver circuit 121Aare illustrated. In addition, in FIG. 17, a clock signal SCK and a startpulse SSP which are supplied from the display control circuit 113 to thesource line side driver circuit 121B are illustrated. Note that, for thedescription of the timing at which the clock signal is output, thewavelength of the clock signal is illustrated with a simple rectangularwave in FIG. 17.

In FIG. 17, potential of Vdd line, potential of the source line 125,potential of the pixel electrode, potential of the terminal 126A,potential of the terminal 126B, and potential of the common electrodeare illustrated.

In FIG. 17, a period 1401 corresponds to a period during which imagesignals for displaying a moving image are written. In the period 1401,operation is performed so that the potential of Vdd line is set to beVdd, the image signal is supplied to each pixel in the pixel portion122, and the common potential is supplied to the common electrode.

A period 1402 corresponds to a period during which a still image isdisplayed. In the period 1402, the potential of Vdd line is set to bethe same potential as the low power supply potential Vss, the supply ofthe image signals and the common potential to the pixels in the pixelportion 122 and the common electrode is stopped. Note that in the period1402 in FIG. 17, each signal is supplied so that the driver circuitportion stops operating; however, it is preferable to write imagesignals periodically in accordance with the length of the period 1402and the refresh rate so as to prevent deterioration of a still image.

First, a timing chart in the period 1401 will be described. In theperiod 1401, the potential of Vdd line is set to be Vdd, a clock signalis supplied all the time as the clock signal GCK, and a pulse inaccordance with a vertical synchronizing frequency is supplied as thestart pulse GSP. In the period 1401, a clock signal is supplied all thetime as the clock signal SCK, and a pulse in accordance with one gateselection period is supplied as the start pulse SSP.

In addition, the image signal Data is supplied to the pixels in each rowthrough the source line 125, and the potential of the source line 125 issupplied to the pixel electrode in accordance with the potential of thegate line 124.

A potential at which the switching element 127 is turned on is suppliedfrom the display control circuit 113 to the terminal 126A of theswitching element 127, and the common potential is supplied to thecommon electrode through the terminal 126B.

On the other hand, the period 1402 is a period during which a stillimage is displayed. Next, a timing chart of the period 1402 isdescribed. In the period 1402, the potential of Vdd line is set to bethe same potential as the low power supply potential Vss, supplies ofthe clock signal GCK, the start pulse GSP, the clock signal SCK, and thestart pulse SSP are all stopped. Further, in the period 1402, the supplyof the image signal Data to the source line 125 is stopped. In theperiod 1402 where the supply of both the clock signal GCK and the startpulse GSP is being stopped, the transistor 214 is off and the potentialof the pixel electrode is in a floating state.

A potential at which the switching element 127 is turned off is suppliedfrom the display control circuit 113 to the terminal 126A of theswitching element 127, and the potential of the common electrode is putin a floating state.

In the period 1402, the potentials of both electrodes of the liquidcrystal element 215, i.e., the pixel electrode and the common electrode,are put in a floating state; thus, a still image can be displayedwithout an additional supply of potential.

The supplies of the clock signal and the start pulse to the gate lineside driver circuit 121A and the source line side driver circuit 121Bare stopped, whereby low power consumption can be achieved.

In particular, a transistor whose off-state current is reduced is usedfor each of the transistor 214 and the switching element 127, whereby areduction over time in the voltage applied to both terminals of theliquid crystal element 215 can be suppressed.

Next, operations of the display control circuit 113 in a period duringwhich a displayed image is switched from a moving image to a still image(a period 1403 in FIG. 17) and in a period during which a displayedimage is switched from a still image to a moving image (a period 1404 inFIG. 17) will be described with reference to FIGS. 18A and 18B. FIGS.18A and 18B illustrate the potential of Vdd line, the clock signal(here, GCK), and the start pulse signal (here, GSP) which are outputfrom the display control circuit 113, and potential of the terminal126A.

The operation of the display control circuit in the period 1403 duringwhich a displayed image is switched from a moving image to a still imageis illustrated in FIG. 18A. The display control circuit stops the supplyof the start pulse GSP (E1 in FIG. 18A, a first step). The supply of thestart pulse GSP is stopped and then, the supply of a plurality of clocksignals GCK is stopped after pulse output reaches the last stage of theshift register (E2 in FIG. 18A, a second step). Then, the potential ofVdd line is set to be the same potential as the low power supplypotential Vss, (E3 in FIG. 18A, a third step). By setting the Vdd lineand Vss line the same potential, potential difference is not generatedin the driver circuit portion 121 which suppress increase of powerconsumption due to leakage of current or the like. After that, thepotential of the terminal 126A is changed to a potential at which theswitching element 127 is turned off (E4 in FIG. 18A, a fourth step).

Through the above steps, the supply of signals to the driver circuitportion 121 can be stopped without malfunction of the driver circuitportion 121. The malfunction occurred when a displayed image is switchedfrom a moving image to a still image causes noise, and the noise is heldas a still image; therefore, a liquid crystal display device thatincludes a display control circuit with fewer malfunctions can display astill image with less image deterioration.

Next, operation of the display control circuit in the period 1404 duringwhich a displayed image is switched from a still image to a moving imagewill be illustrated in FIG. 18B. The display control circuit changes thepotential of the terminal 126A into a potential at which the switchingelement 127 is turned on (S1 in FIG. 18B, a first step). Then, thepotential of Vdd line is changed from the same potential as the lowpower supply potential Vss to the high power supply potential Vdd (S2 inFIG. 18B, a second step). After that, high potential is supplied as theclock signal GCK which is a pulsed signal longer than a regular clocksignal GCK which is supplied later and then, a plurality of clocksignals GCK are supplied (S3 in FIG. 18B, a third step). Next, the startpulse signal GSP is supplied (S4 in FIG. 18B, a fourth step).

Through the above steps, the supply of driving signals to the drivercircuit portion 121 can be resumed without causing malfunction of thedriver circuit portion 121. Potentials of the wirings are sequentiallychanged back to those at the time of displaying a moving image, wherebythe driver circuit portion can be driven without malfunction.

FIG. 19 schematically illustrates writing frequency of image signals inframe periods in a period 601 during which a moving image is displayedand in a period 602 during which a still image is displayed. In FIG. 19,W indicates a period in which an image signal is written, and Hindicates a period in which the image signal is held. In addition, aperiod 603 is one frame period in FIG. 19; however, the period 603 maybe a different period.

As described above, in the structure of the liquid crystal displaydevice of this embodiment, an image signal of a still image displayed inthe period 602 is written in the period 604, and the image signalwritten in the period 604 is maintained in the other periods of theperiod 602.

Here, operation of displaying a moving image and a still image of aliquid crystal display device having the pixel structure illustrated inFIG. 15 is described with reference to FIG. 17, FIGS. 18A and 18B, andFIG. 19. The same can be said to a liquid crystal display device havingthe pixel structure illustrated in FIG. 16. The pixel 136 in FIG. 16includes the pixel 136 a having a reflective pixel electrode and thepixel 136 b having a light-transmitting pixel electrode. One or both ofthe pixel 136 a and the pixel 136 b can be used for displaying a stillimage or a moving image. In other words, when the term “the pixel 123”is replaced with the term “the pixel 136”, “the pixel 136 a”, or “thepixel 136 b”, operation of displaying a still image and a moving imageof a liquid crystal display device can be described in the same way asthat of the liquid crystal display device in FIG. 15.

The liquid crystal display device described in this embodiment as anexample can decrease writing frequency of an image signal in a periodduring which a still image is displayed. As a result, power consumptionat the time of displaying a still image can be reduced.

When a still image is displayed by rewriting the same image pluraltimes, eye strain may be caused when switching of images is recognized.In the liquid crystal display device of this embodiment, writingfrequency of an image signal is reduced, which is effective in reducingeye strain.

Specifically, in the liquid crystal display device of this embodiment,transistors whose off-state currents are reduced are used for pixels anda switching element of the common electrode, whereby a period (time) ofholding voltage in a storage capacitor can be longer. As a result,writing frequency of an image signal can be extremely reduced, which issignificantly effective in reducing power consumption and eyestrain whena still image is displayed.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

Embodiment 5

In this embodiment, an example of an electronic device including theliquid crystal display device described in any of Embodiments 1 to 4will be described.

FIG. 20A illustrates an electronic book reader (also referred to as ane-book reader) which can include housings 9630, a display portion 9631,operation keys 9632, a solar battery 9633, and a charge and dischargecontrol circuit 9634. The electronic book reader is provided with thesolar battery 9633 and a display panel so that the solar battery 9633and the display panel can be opened and closed freely. In the electronicbook reader, power from the solar cell is supplied to the display panelor a video signal processing portion. The electronic book readerillustrated in FIG. 20A can have a function of displaying various kindsof data (e.g., a still image, a moving image, and a text image), afunction of displaying a calendar, a date, the time, or the like on thedisplay portion, a touch-input function of operating or editing theinformation displayed on the display portion by touch input, a functionof controlling processing by various kinds of software (programs), andthe like. Note that in FIG. 20A, a structure including a battery 9635and a DCDC converter (hereinafter abbreviated as a converter 9636) isillustrated as an example of the charge and discharge control circuit9634.

The display portion 9631 is a reflective liquid crystal display devicehaving a touch-input function with the use of photo sensors and is usedin a comparatively bright environment. Therefore, the structureillustrated in FIG. 20A is preferable because power generation by thesolar battery 9633 and charge in the battery 9635 can be performedeffectively. Note that a structure in which the solar battery 9633 isprovided on each of a surface and a rear surface of the housing 9630 ispreferable in order to charge the battery 9635 efficiently. When alithium ion battery is used as the battery 9635, there is an advantageof downsizing or the like.

The semi-transmissive liquid crystal display device described inEmbodiment 1 is used in the display portion 9631, so that an electronicbook reader with bright and high-quality display can be realized.

The structure and the operation of the charge and discharge controlcircuit 9634 illustrated in FIG. 20A are described with reference to ablock diagram in FIG. 20B. FIG. 20B illustrates the solar battery 9633,the battery 9635, the converter 9636, a converter 9637, switches SW1 toSW3, and the display portion 9631. The battery 9635, the converter 9636,a converter 9637, and switches SW1 to SW3 correspond to the charge anddischarge control circuit 9634.

First, an example of operation when power is generated by the solarbattery 9633 using external light is described. The voltage of powergenerated by the solar battery is raised or lowered so that the powerhas a voltage for charging the battery 9635. Then, when the power fromthe solar battery 9633 is used for the operation of the display portion9631, the switch SW1 is turned on and the voltage of the power is raisedor lowered by the converter 9637 so as to be a voltage needed for thedisplay portion 9631. In addition, when display on the display portion9631 is not performed, the switch SW1 is turned off and a switch SW2 isturned on so that charge of the battery 9635 may be performed.

Note that although the solar battery 9633 is described as an example ofa means for charge, charge of the battery 9635 may be performed withanother means. Alternatively, a combination of the solar battery 9633and another means for charge may be used.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

This application is based on Japanese Patent Application serial No.2010-043185 filed with Japan Patent Office on Feb. 26, 2010, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. An active matrix display device comprising: abacklight configured to emit light; and a pixel portion over thebacklight, the pixel portion comprising: a first layer; a second layerover and in contact with the first layer; a reflective layer over and incontact with the second layer; a reflective electrode comprising anopening; and a liquid crystal element over the reflective electrode,wherein the opening is located over the first layer, the second layer,and the reflective layer, wherein a refractive index of the first layeris different from a refractive index of the second layer, wherein thesecond layer includes a first side and a second side opposite the firstside, wherein the reflective layer includes a first surface in contactwith the first side of the second layer, wherein the reflective layerincludes a second surface in contact with the second side of the secondlayer, wherein the first layer is provided between the first surface andthe second surface of the reflective layer, and wherein the opening isconfigured to transmit light.
 2. The active matrix display deviceaccording to claim 1, wherein a shape of the opening is square.
 3. Theactive matrix display device according to claim 1, wherein the openingis surrounded by parts of the reflective electrode.
 4. The active matrixdisplay device according to claim 1, wherein the reflective electrodecomprises a curving surface with an uneven shape.
 5. The active matrixdisplay device according to claim 1, wherein the reflective electrodecomprises a plurality of conductive films.
 6. The active matrix displaydevice according to claim 1, wherein the reflective electrode isconfigured to reflect light transmitted through the liquid crystalelement.
 7. The active matrix display device according to claim 1,wherein the reflective electrode comprises at least one of silver andaluminum.
 8. The active matrix display device according to claim 1,further comprising a transparent electrode overlapping with the opening.9. The active matrix display device according to claim 1, furthercomprising an insulating layer comprising an organic resin, wherein thereflective electrode is provided over the insulating layer.
 10. Anactive matrix display device comprising: a pixel portion comprising: afirst layer; a second layer over and in contact with the first layer; areflective layer over and in contact with the second layer; a reflectiveelectrode comprising a plurality of openings; and a liquid crystalelement over the reflective electrode, wherein the plurality of openingsis located over the first layer, the second layer, and the reflectivelayer, wherein a refractive index of the first layer is different from arefractive index of the second layer, wherein the second layer includesa first side and a second side opposite the first side, wherein thereflective layer includes a first surface in contact with the first sideof the second layer, wherein the reflective layer includes a secondsurface in contact with the second side of the second layer, wherein thefirst layer is provided between the first surface and the second surfaceof the reflective layer, and wherein the plurality of openings isconfigured to transmit light.
 11. The active matrix display deviceaccording to claim 10, wherein a shape of each of the plurality ofopenings is square.
 12. The active matrix display device according toclaim 10, wherein each of the plurality of openings is surrounded byparts of the reflective electrode.
 13. The active matrix display deviceaccording to claim 10, wherein the reflective electrode comprises acurving surface with an uneven shape.
 14. The active matrix displaydevice according to claim 10, wherein the reflective electrode comprisesa plurality of conductive films.
 15. The active matrix display deviceaccording to claim 10, wherein the reflective electrode is configured toreflect light transmitted through the liquid crystal element.
 16. Theactive matrix display device according to claim 10, wherein thereflective electrode comprises at least one of silver and aluminum. 17.The active matrix display device according to claim 10, furthercomprising a transparent electrode overlapping with the plurality ofopenings.
 18. The active matrix display device according to claim 10,further comprising an insulating layer comprising an organic resin,wherein the reflective electrode is provided over the insulating layer.19. An active matrix display device comprising: a thin film transistorover an insulating surface; a first layer over the insulating surface; asecond layer over and in contact with the first layer; a reflectivelayer over and in contact with the second layer; a reflective electrodeelectrically connected to the thin film transistor, the reflectiveelectrode comprising an opening; and a liquid crystal element over thereflective electrode, wherein the opening is located over the firstlayer, the second layer, and the reflective layer, wherein a refractiveindex of the first layer is different from a refractive index of thesecond layer, wherein the second layer includes a first side and asecond side opposite the first side, wherein the reflective layerincludes a first surface in contact with the first side of the secondlayer, wherein the reflective layer includes a second surface in contactwith the second side of the second layer, wherein the first layer isprovided between the first surface and the second surface of thereflective layer, and wherein the opening is configured to transmitlight.
 20. The active matrix display device according to claim 19,wherein the opening is surrounded by parts of the reflective electrode.21. The active matrix display device according to claim 19, wherein thereflective electrode is configured to reflect light transmitted throughthe liquid crystal element.
 22. The active matrix display deviceaccording to claim 19, further comprising a transparent electrodeoverlapping with the opening.
 23. The active matrix display deviceaccording to claim 19, further comprising an insulating layer comprisingan organic resin, wherein the reflective electrode is provided over theinsulating layer.